Answer:
b) remains
Step-by-step explanation:
"Remains" is the grammatically correct word to occupy the white space shown in the question above.
That's because that word is related to the word "Optimism", which represents the pronoun "it" in that sentence. As we know, when using the pronouns "he, she, it" we must conjugate the verbs. The correct form of "remain" is "remains".