Question

—Are you still busy?—Yes.I___the report for the manager and it will not take long.

B.am just finishing D.am just going to finish选哪个？

Answer 1

B is better。
I am just finishing the report. This means that you have been working on something and are still working on it. It fits well with "Are you still busy?"
I am just going to finish the report. "Just going to finish" suggests that you are about to do something that you weren't doing before. It doesn't fit as well with "Are you still busy?"

Answer 2

B,现在进行时表将来。

追问

D不可以吗？

追答

用法是这样，D也行

Answer 3

b,d都可以！