Answer:
A) phrase
B) phrase
C) clause
Step-by-step explanation:
First we need to make a distinction between a phrase and a clause.
A phrase is a part of sentence, a set of words, that lacks in subject, verb or both so it can never stand on its own. It can have several functions in a sentence depending on the piece of information it carries.
A clause is a part of a sentence that has a subject and a verb and can either stand on its own as a complete sentence (independent clause) or not (dependent clause).
A) This is the phrase because it doesn't have a subject and "walking" isn't a verb, but a gerund. Most likely it's an adverbial phrase telling how or why something happened. For example:
Quickly walking to the store, Ian sprained his ankle.
B) This is also a phrase because it only gives us a time frame; we don't know who will (or won't) perform an action nor what will (or won't happen). For example:
I won't leave the house until next week.
C) This is a clause; it has subject (they) and verb (tried). However it doesn't make a complete sentence, so it's a dependent clause:
They were exhausted yet they tried.