Which words make up the adverb phrase in this sentence?
before the weekend, we must finish our project.
a. before the weekend
b. we must
c. our project
d. must finish

The adverb phrase here is "before the weekend". 

Adverb phrases describe how, when or where something happens, similarly to adjectives: they have similar functions. 

In the phrase above it gives an information is about when "the finishing of the project:" happened.