Why is the answer this?

Hello

I'd like to ask why the answer is (b) in the following question.

There was the question #1 in the exercise to fill in the blank.
#1 The function of space suit is ( ).
Choices are (a) keeping the wearer safe (b) to keep the wearer safe

The answer was (b).
I cannot understand why (a) is wrong. A gerund can be used as a noun. So I think both can be the right answer.
Is there a special rule that gerund cannot be a complement?
Would you give me some guidance?

Thank you.

[Tdol]

When talking about purpose, we tend to use an infinitive with a verb and 'for' with a noun.