Question

Several of the girls had forgotten to bring _____ equipment. Which pronoun agrees with the underlined indefinite pronoun?

Answer 1

The answer is : Their

Answer 2

Several of the girls had forgotten to bring their equipment.

It's not ones equipment because there is more than one girl.
It's not her equipment because there is more than one girl.
It's not his equipment because they are girls (feminine).

So the answer is "their", because it corresponds with several.