The correct option is
A finish
The given statement is in the form of the first conditional. A first conditional talks about something that might happen in the future. Its structure is:
if + simple present verb,... will + infinitive
As we can see from the above structure, the blank must contain a simple present verb.
Option A is correct. 'finish' is a verb in the simple present tense. Thus, it fits in the given structure and is the correct answer.
Option B is incorrect. 'will finish' is in the simple future tense and not simple present tense. Thus, it does not fit in the structure.
Option C is incorrect. 'are going to finish' is in the present continuous tense and not simple present tense. Thus, it does not fit in the structure.
Option D is incorrect. 'finished' is in the simple past tense and not simple present tense. Thus, it does not fit in the structure.