"I have chosen," "I choose," or "I am choosing"?
Solution 1:
I think you have to take your lead from the question. You have to decide which outfit would suit you best.
Solution 2:
You could go with any of the three, as well as others. Moreover, despite what your teacher says, it is grammatical English to use any of the following:
I choose X.
I chose X.
I have chosen X.
I am choosing X.
I will choose X.
These all suggest that you are relating to another party a choice that you have made. A native English speaker would understand any of them to mean that you are announcing your decision.