"His corporation has a large investment fund" should probably be accepted
What do you mean, "probably? " It most certainly should be accepted.
I would say: "His corporation has a large investment fund"
three words are listed for "lon:" big, large and great. Only one of them would be correctly used in this sentence, and it is NOT the one listed in the answer; it is the one Duo marked wrong!
'Large' - not accepted.