The problem with this question's answers is that all of the answers are grammatically correct. However, the only real way to choose is based on context. The answer would be A if the person was doing something in the garden previously to be asked the question. The answer would be B if the person was getting paid to do some work in the garden. The answer would be C if the person was and still is working in the garden when being asked the question. With that information I'm gonna guess definitely not C because the person wouldn't ask a question like that if they're with them in the garden. But the other two seem completely viable.

It is so subtle!!

Yes, the solution says the answer is C. I thought it was a mistake because I chose A. Thanks Peachey for explaining why C has to be the answer!

The only correct answer is C. The question is about the present, so the reply must use a present tense - in this case, present perfect. The confusion lies in the habits of some speakers to cut corners and use past tenses. The past tense answers are definitely understandable, but not correct.

I choose A because question was made in present tense and answer must be in the same verbal time. Letter 8 is wrong because it a contiinuous tense, meaning a non stop action and letter b is past tense