Which is better in this sentence, "maintain" or "continue"?

"If a conversation starts angry, it will almost certainly _____ angry."

This is a question from a cloze quiz, the two options are "maintain" or "continue".

The answer key says that "continue" is the correct answer, but I never heard someone say:"continue adj.", only "continue to be adj." I do however believe that "maintain adj." is a valid structure.

Which is the real correct one, and why?

Continue sounds better in that sentence but note that that sentence does not represent idiomatic standard English. Instead it should be something such as continue in an angry manner. Or continue angrily. The use of the 'flat adverb' angry is okay but might not represent standard English. For example, If a conversation starts angrily, it will almost certainly finish/continue angrily represents standard English, whereas If a conversation starts angry, it will almost certainly finish angry sounds dialectal/quaint/country.

As for maintain, it's incorrect in any of these sentences because maintain is used as a transitive verb and so requires an object such as a direct object or a reflexive object (maintains itself angrily, although this is most unnatural).

The correct one is continue.

The difference is subtle, but 'maintain' means:

cause or enable (a condition or state of affairs) to continue.

This is wrong because when the conversation starts angry, it is not causing it to continue angry. You'd say that the people having the angry conversation maintained it (without them, it wouldn't continue angry)

Whereas 'continue' is simply:

persist in an activity or process.

Which is right, because the angry conversation is persisting.

The reason that maintain is wrong is that maintain is a transitive verb: it must take an object, something which is maintained by a positive action.

Continue can be transitive, but it can also be intransitive. Something may simply continue; it doesn't have to be continued.

Thus the conversation continues.

The use of an adjective after continue rather than an adverb is dialectal.