"Charles" schreef
Two meanings for non-flowering. One is plants that never flower,
ferns, mosses, and the like, the other meaning would be plants that
don't have flowers at the time you observed them. Do you know which
meaning is intended?

***
Your observation about "flowering" is correct, but your question is not. As
the phrase "non-flowering plants" is qualified by "(NOT trees)", it cannot
refer to mosses. A much better question here is after the meaning of
"plants" as used here.

Therefore, there are two possible keys to decipher this assignment:
I. the "(NOT trees)" qualifies the entire phrase and
"plants" = herbaceous plants, belonging to the angiosperms
"non-flowering" = in the vegetative state

II or alternatively in "non-flowering plants" it is only the latter word
that is qualified by the "(NOT trees)"
"plants" = herbaceous plants
"non-flowering" = not in flower, either in the vegetative state or a
non-angiosperm

It is likely that in judging the incoming results the teacher will tolerate
either interpretation and accept, say, a fern as non-flowering plant.
PvR