Generic nouns refer to a specific person, place, or thing.
a. always
b. sometimes
c. don’t
d. usually

I would say that generic nouns don't refer to a specific person, place, or thing. The nouns which do that are called definite, or collective nouns.
Generic nouns refer to some person, some place, some group, as in this example: Cats are animals.
It isn't a specific group, it is all cats.