What is the plural of the noun ‘stimulus’?

The correct answer is Option (2) → Stimuli

Plural of nouns ending in -us (from Latin):

Some nouns that came into English from Latin end in -us. They change -us to -i in the plural. So stimulus → stimuli. A stimulus is something that makes a living thing react. “Stimula” is wrong because the -a ending belongs to nouns that end in -um.