yoyakutorikesi ka daitairuuto wo erabu.

Because 予約取消か代替ルート is being treated as one combined set of alternatives: reservation cancellation or an alternative route. The particle を marks that whole thing as the object of 選ぶ.

So the structure is:

• 予約取消か代替ルート = cancellation or alternative route
• を選ぶ = choose

This is very common in Japanese. When A か B means A or B, you usually put the case particle only once, after the whole pair.

Here, か means or.

So:

• 予約取消 = reservation cancellation
• 代替ルート = alternative route
• 予約取消か代替ルート = reservation cancellation or an alternative route

This is a very basic and common use of か in Japanese.

Yes, in fuller Japanese, 予約の取消 is very natural.
What you see here, 予約取消, is a compact compound noun style often used in signs, menus, buttons, system messages, forms, and business Japanese.

So these are related:

• 予約取消 = compact label-like form
• 予約の取消 = fuller noun phrase
• 予約を取り消す = verbal expression, cancel the reservation

This kind of shortened style is especially common in interfaces and official-looking text.

取消 is a more compressed, formal, kanji-heavy way of writing 取り消し.

Both refer to cancellation, but the feeling is different:

• 取り消し: more standard everyday writing
• 取消: more formal, technical, or label-like

In UI text, notices, and bureaucratic language, Japanese often drops okurigana and uses a tighter kanji form. So 予約取消 looks like something you might see in a ticketing system, website menu, or transport app.

It is usually read とりけし here.

So 予約取消 would normally be understood as よやく とりけし in this kind of context.

However, learners should know that this is a shortened written form of 取り消し, so the connection to the verb 取り消す is important.

It is grammatically understandable, but it has a short, instruction-like feel rather than the feel of a full conversational sentence.

選ぶ is the plain dictionary form to choose, but in context it can function like:

• choose ...
• select ...
• please choose ...

In interface Japanese, headings and commands are often shortened like this. A fuller version might be:

• 予約取消か代替ルートを選んでください。
• 予約の取消か代替ルートを選ぶ。 depending on context

So yes, it can feel abrupt, but that is normal in system/menu language.

Because dictionary form is often used in:

• menu labels
• button instructions
• manuals
• procedures
• system prompts

It gives a neutral, compact instruction. Japanese often avoids full polite forms when space is limited or when the text is functioning more like a label than a spoken sentence.

Compare:

• 予約取消か代替ルートを選ぶ。 = compact / interface-like
• 予約取消か代替ルートを選んでください。 = explicit polite request
• 予約取消か代替ルートを選択してください。 = more formal/system-like

It sounds somewhat stiff and administrative, but it can be natural in the right context.

A native speaker would be more likely to say something like:

• 予約を取り消す
• 予約の取り消し
• キャンセル

in everyday conversation.

But in a train app, booking page, help screen, or official form, compact terms like 予約取消 are quite plausible.

So the key point is: it is more natural in written system language than in casual speech.

Yes, very often.

In many everyday contexts, Japanese speakers use キャンセル very naturally:

• 予約をキャンセルする
• キャンセルする
• 予約のキャンセル

Compared with 取消, キャンセル often sounds more everyday and modern.
取消 sounds more formal, institutional, or technical.

Which one is used depends a lot on the situation and the style of the text.

It is a noun meaning alternative route.

Breakdown:

• 代替 = substitution / alternative
• ルート = route

So 代替ルート is a route used instead of the original one.

This kind of word is common in transportation, mapping, logistics, and travel contexts.

Yes, 代替 is a word learners often notice because there are different pronunciations in real life.

The standard dictionary reading is often given as だいたい, but だいがえ is also very commonly heard in everyday speech. In compounds, usage can vary depending on the word and speaker.

For 代替ルート, many people would understand it regardless of which common pronunciation is used, but learners should be aware that pronunciation variation exists.

The written form strongly suggests that 予約取消 is being treated as a noun option, not as a verb phrase.

So the structure is:

• [予約取消] か [代替ルート] を選ぶ
• choose [reservation cancellation] or [alternative route]

If the meaning were cancel the reservation or choose an alternative route, Japanese would more likely use two verbal phrases, such as:

• 予約を取り消すか、代替ルートを選ぶ

That version clearly presents two actions.

Both are possible.

• 選ぶ = basic, common verb choose
• 選択する = more formal / technical select

In software, forms, and official writing, 選択する is very common.
Using 選ぶ makes the wording a little more direct and basic.

So:

• 予約取消か代替ルートを選ぶ。 = simpler
• 予約取消か代替ルートを選択する。 = more formal/system-like

Yes. Normal Japanese writing usually does not use spaces between words.

So the standard written form would be:

予約取消か代替ルートを選ぶ。

The spaces in learning materials are often added only to help beginners see the parts more easily.