Right Answer is:
Insure- secure or protect someone against (a possible contingency).
Ensure- make sure that (a problem) does not occur.
Assume- suppose to be the case, without proof.
Assure- tell someone something positively to dispel any doubts.
In the sentence, a person is being optimistic about the success in the interview. So, the word “assured” fits best in the context.
Hence, option D is the correct answer.