It's hard to answer without knowing the vocabulary. I think Duolingo should have some vocabulary lists in between lessons.
If you did it with the word bank or the three-choices, you can intuit which it is because of the grammar. Eg. matching feminine to feminine etc.
This is very helpful and Wilbur111 is correct....
So I wrote लम्बा instead of लंबा and it was accepted. Is it always okay to turn the nasalization into a conjunct consonant like that? Like सन्दर <-> संदर ?