If someone sees an accident, __________ should call an ambulance.

The correct answer is: He or she

Personal pronouns are words used in place of nouns referring to persons. 'I, We, You, He, She, It, They'. She and he is third person singular number. She and he is a word that takes place of singular persons. She, he and they are used when speaking about someone or something else
'If someone sees an accident, they should call an ambulance. Here, 'someone' is also third person singular number that's why 'they' is the correct answer.