Answer:
3. certitude
Step-by-step explanation:
Certitude makes the most sense definition wise and the way it flows with the sentence.
Ascertion means to "find (something) out for certain; make sure of." In this sentence, the speaker has implied that they are sure about something, not unsure. Ascertion wouldn't make sense.
Certified doesn't sound right in the sentence, it's an adjective, which doesn't fit in the sentence.
Uncertain doesn't sound grammatically right either.
Thus, option 3 is the best answer.
hope this helps!