her ultimate success was surprising to us.\na unknown\nb failure\nc optimistic\nd eventual

Brief Explanations:

"ultimate" means final or eventual. We need to find a word with a similar meaning among the options. "unknown" means not known, "failure" is the opposite of success, "optimistic" is related to a positive - thinking attitude. "eventual" means happening at the end of a process or period of time, similar to "ultimate".

Answer:

D. eventual