- My hands are muddy because I __________ some flowers.
a. have planted
b. have been planting
Hi, Hussein: I agree with your book that the present-perfect progressive is needed there: "My hands are muddy because I have been planting flowers." ("Some" is unnecessary.) The idea is that the action of planting flowers continued right up until the present moment, and that is why your hands are muddy. The present-perfect progressive communicates that the action continued right up until the present moment.
"I have planted some flowers" could refer to something you did last week. In any case, it needn't refer to something you just did. It is not impossible to use the non-progressive present perfect there; however, an adjustment would be needed. You would need to use "just": "My hands are muddy because I have just planted some flowers." Although that sentence is correct, I would more naturally use the present-perfect progressive.