Answer:
The correct answer is A. positive.
Step-by-step explanation:
Positive is the most basic form of adjectives and adverbs - it is the form we use when we are stating facts, describing someone, and not comparing them. For example: beautiful, nice, smart. This is the case for the adjective above - optimistic is the basic form.
If the sentence said - He is always more optimistic than him/He is the most optimistic person I know, then those would be comparative and superlative, respectively. And since we can say that someone is more optimistic/most optimistic, it means that the adjective is comparable; therefore, D is incorrect.