The correct option is
C At last
Answer : B
Explanation:
First, let us get familiar with the meanings of the words in the options:
--together means : companionship / association
--At last means : finally / in the end
--Exactly means : Precisely / absolutely / completely / to confirm or agree
--Just means : recently / simply / exactly / barely / expressing agreement
Reading the sentence, we know that the speaker has "finished" his homework, that it is completed.
The suitable expression to complete the given sentence with is "At last",
which means that finally / in the end the speaker completed his homework and it makes sense. After putting in the efforts and despite finding it "boring", the speaker has finally completed it.
So basically the activity that the speaker was involved in is now over.
"Together" here does not make sense and therefore is inappropriate for use here.
"Exactly" again in inappropriate for use because when placed in the beginning of this sentence it is grammatically incorrect. Though it could be used immediately before the word "finished".
"Just" could have been used had the blank been before the word "finished", but when used in the beginning of THIS SENTENCE; it would be grammatically incorrect.
Therefore the correct complete sentence is:
AT LAST I have finished this exercise. Thank goodness! It was so boring.