Answer:
The suffix -ist in the word optimist means a person who practices optimism.
Step-by-step explanation:
In the nouns ending with -ism, which is the case of optimism, pesimism, traditionalism, alcoholism, etc. when the suffix -ist is replacing -ism it is used to speak of a person who practices what the noun is refering to.
In the case of optimist, it refers to someone who lives with optimism.