Marie choisit la huitième photo.

Because choisit is the conjugated form of the verb choisir for Marie.

• choisir = the infinitive, to choose
• choisit = she chooses or she is choosing

Since Marie is third-person singular, French uses choisit in the present tense.

It can mean either one, depending on context.

French often uses the simple present where English might use either:

• Marie choisit la huitième photo. = Marie chooses the eighth photo.
• Marie choisit la huitième photo. = Marie is choosing the eighth photo.

If you specifically want to stress is in the process of choosing, French can also use être en train de, but the basic present tense is very common.

Because la means the, not a.

Here, the sentence refers to a specific photo: the one that is eighth in the set. That makes it definite, so French uses la.

• la huitième photo = the eighth photo
• une huitième photo would mean an eighth photo, which usually does not fit the same idea

Also, photo is a feminine noun, so the definite article is la.

Because huitième is an ordinal number, while huit is a cardinal number.

• huit = eight
• huitième = eighth

In English, this is the difference between eight photos and the eighth photo.

So:

• la huitième photo = the eighth photo
• huit photos = eight photos

Because ordinal numbers in French usually come before the noun.

So French says:

• la huitième photo
• le troisième chapitre
• la première fois

This is similar to English, which also says the eighth photo, not the photo eighth.

Because although the h in huitième is silent, the word begins with the sound /ɥ/, a consonant-like glide sound, not a simple vowel sound.

So French keeps la:

• la huitième photo

not

• l'huitième photo

This is one of those pronunciation-based spelling patterns that learners simply need to get used to.

Yes, adjectives in French agree with the noun, and ordinal numbers act like adjectives.

Here:

• photo is singular
• huitième is singular too

For gender, huitième has the same form in the masculine and feminine singular, so you do not see a spelling change here.

Compare:

• le huitième livre
• la huitième photo

The article changes, but huitième stays the same.

It is both a real, normal French word and a shortened form of photographie.

In everyday French, photo is extremely common and completely standard.

So:

• une photo = normal everyday usage
• une photographie = more formal or more full-length

Most speakers would naturally say photo here.

choisit is pronounced approximately shwa-zee.

A few useful points:

• the ch sounds like sh
• the final t is silent
• the stress is not as strong as in English

So although it is spelled choisit, you do not pronounce the final t.

A rough English-style pronunciation is:

ma-REE shwa-ZEE la nwee-TYEM fo-TO

More closely in IPA:

/ma.ʁi ʃwa.zi la ɥi.tjɛm fɔ.to/

A few notes:

• Marie = ma-ree
• choisit = shwa-zee
• huitième begins with a rounded sound /ɥ/ that English does not really have
• photo is pronounced fo-to, with both syllables clearly heard

Because French, like English, can use a subject noun directly before the verb.

Here:

• Marie = the subject
• choisit = the verb

So Marie choisit means Marie chooses / is choosing.

French does not need an extra subject pronoun like elle here, because Marie already tells us who is acting.