I went with "It's understood by me now" and was marked wrong. Who in the world would say "It's understood to me now" instead of "I understand now". The exercise is forcing a passive voice where none would ever be so if course it's difficult to translate
The sentences "זה מובן לי" and "אני מבין את זה" mean pretty much the same, both are correct in my opinion.
However, I think this Hebrew sentence isn't quite right. I believe a subject is missing - what is understood. It's not required in the active "אני מבין" (literally - I understand), but is in "מובן לי" (literally - understood to me). Whether it's "זה מובן לי", or "מובן לי הדבר", or something else of this sort.
"Do you understand the answer?" = "?אתה מבין את התשובה "
Two possible responses:
1) "I understand now." = "אני מבין עכשיו". But not "מובן לי עכשיו" or "מובנת לי עכשיו", both sound very awkward.
2) "I understand it now" = "אני מבין אותה עכשיו" or "היא מובנת לי עכשיו"
But I'd like to hear what others think.
I commented on a different version of this question, and agree completely that Duo has missed the boat on this one. ("Missed the boat" is an idiom that means "they have it all wrong.")