Suppose I took a test with 100 questions, and I told you:
I was unsuccessful in getting only 15 questions wrong.
What does that mean? If I got 86 questions right, 14 wrong, then was I 'unsuccessful in getting only 15 questions wrong?' (I'm interpreting 'only' as 'exactly' here, which is the part I'm unsure of).
For comparison, the original answer choice was
collection agencies that are assigned bills for collection by companies are unsuccessful in collecting, on average, only 15 percent of the total amount of those bills
Ultimately this one could've been eliminated for other reasons, but I'm still curious as to what its meaning is.