Apples are one of my favorite fruits,

Apples are one of my favorite fruits,

Using fruits
When it refers to "fruit", it is a collective noun without s
Fruit refers to the result of different kinds of fruits or things
Apple is one of my favorite fruits. There are different kinds of fruits here. It refers to different kinds of fruits
So you should use fruits

the apple is a kind of ______ A fruit B fruits

A. Fruit is plural only when it emphasizes many varieties or chooses among many varieties

the monkeys___ interesting.is kind of
a. is kind of b. are kinds of c. are kind of

C
Kinds of
Kind of
The monkeys is plural, so they are
The monkeys are a little bit interested