Translation:Of course I did not tell him.
A more direct translation for "obviously" is 明显的/地, and for "definitely", 肯定/一定/铁定.
I'd like to suggest that no capital letters be used in the list of English word choices except where grammatically necessary so that the student must decide which word opens the sentence. The app always capitalizes the first word in the sentence, in effect giving the answer.
If you don't capitalize one word then you will end up with more syntax variation and complaints about why your sentence wasn't accepted. In this particular sentence my first impulse was to say "I didn't tell him, of course" and I'm sure that would have been rejected.
As long as you remember to put a comma after "him". :) "I did not tell him, of course."