Learn Hardcore Norwegian: Alternativet med buss er bedre i dag.

Because alternativ is a neuter noun. Indefinite singular is et alternativ; definite singular is alternativet. The sentence refers to a specific, known option, so the definite form is used. Useful forms:

et alternativ (indefinite singular)
alternativet (definite singular)
alternativer (indefinite plural)
alternativene (definite plural)

Why is it med buss? Could it be på or i instead?

For modes of transport, Norwegian normally uses med (by/with): med buss, med tog, med bil, med fly, med båt, med trikk.
Use i/på only when you mean physically in/on a specific vehicle:

i bussen = inside the bus
på bussen = on the bus (on board)
Not på buss for the general mode.
Special cases: på sykkel (by bike), til fots (on foot).

Why not med bussen?

med buss = by bus in general (the bus option as a mode of travel).
med bussen = with the (particular) bus; it can refer to a specific bus/route/trip, e.g., Jeg drar med bussen klokka 8. In everyday speech some people also use the definite form generically, but med buss is the neutral, textbook way to express the mode.

Can I say Bussalternativet er bedre i dag or Det er bedre å ta buss i dag instead?

Yes.

Bussalternativet er bedre i dag uses a compound noun and is very natural.
Det er bedre å ta buss i dag focuses on the action and is often the most idiomatic way to give advice.

Why is i dag at the end? Can it come first?

Time adverbials often go near the end, but you can front them. Norwegian main clauses have V2 word order (the verb comes second), so:

I dag er alternativet med buss bedre. (fronted time; verb still second) The original Alternativet med buss er bedre i dag. is also perfectly normal.

Can I put i dag in the middle: Alternativet med buss er i dag bedre?

Yes. That order is possible and can sound a bit more formal or contrastive. All three are fine:

I dag er alternativet med buss bedre.
Alternativet med buss er i dag bedre.
Alternativet med buss er bedre i dag.

Why bedre and not something like mer bra? Does bedre change form?

bedre is the irregular comparative of bra/god. Use:

bra/god → bedre → best/beste
Comparatives like bedre do not change for gender or number, so you don’t add endings: Alternativet er bedre, regardless of neuter/plural.

How do I say “better than …” in Norwegian?

Use enn:

Alternativet med buss er bedre enn alternativet med tog.
Alternativet med buss er bedre enn i går.

Does the present tense here imply the future? How would I say “will be better”?

Norwegian often uses the present with a time expression for near-future facts, so … er bedre i dag can be understood as a present-day assessment or a plan-related statement. To be explicit about change or futurity:

… blir bedre i dag (becomes/ends up better today)
… vil være/kommer til å være bedre i dag (will be better today)

Is i dag one word or two?

Two words: i dag. Writing idag is nonstandard in modern Bokmål (and Nynorsk also uses i dag).

Could I drop med and say Alternativet buss?

No. You need either the preposition (alternativet med buss), a compound noun (bussalternativet), or a clause:

Det er bedre å ta buss i dag.
Å ta buss er et bedre alternativ i dag.

Does bedre need a neuter ending because alternativ is neuter?

No. Comparative adjectives like bedre don’t inflect. But in the positive and superlative you would see agreement/variation:

et godt/bra alternativ
det beste alternativet

Is bedre ever a verb?

Yes, å bedre means “to improve” (verb): å bedre – bedrer – bedret – har bedret. In the sentence, bedre is the comparative adjective, not the verb.

Can I move med buss elsewhere? Does it change the meaning?

Bussalternativet er bedre i dag. (clear: the bus option is better)
Alternativet er bedre med buss i dag. can sound like “with a bus, the option is better (today),” which may be read as a condition rather than a label. If you mean the “bus option,” prefer alternativet med buss or bussalternativet.

Any quick pronunciation tips?

alternativet: stress on the na syllable (al-ter-NA-ti-ve), final -et is a schwa-like sound.
buss: short front u (like French u/German ü), with a long ss.
i dag: roughly “ee dahg”; many drop or soften the final g in dag. Dialects vary.