A noun or noun phrase should not be followed by an infinitive.

What are you doing buddy, can you post actual question with all options?

saurabhmahajan wrote:For members of the ashanti nation in africa, animal-hides shields with wooden frames were essential items of military equipment , a method to protect warriors against enemy arrows and spears.

well the answer is use protecting in place of underlined words.
my question is why cant we use to protect instead.?

saurabhmahajan wrote:For members of the ashanti nation in africa, animal-hides shields with wooden frames were essential items of military equipment , a method to protect warriors against enemy arrows and spears.

well the answer is use protecting in place of underlined words.
my question is why cant we use to protect instead.?

Here protecting is an adverbial modifier.It modifies the previous clause.
We can't use an infinitive "to protect" because of the comma before it.

saurabhmahajan wrote:For members of the ashanti nation in africa, animal-hides shields with wooden frames were essential items of military equipment , a method to protect warriors against enemy arrows and spears.

well the answer is use protecting in place of underlined words.
my question is why cant we use to protect instead.?

Don't put the half information. It can mislead to you and others,

Here is the complete question :

For members of the seventeenth century Ashanti nation in Africa, animal-hide shields with wooden frames were essential items of military equipment, a method to protect warriors against enemy arrows and spears.

A) a method to protect
B) as a method protecting
C) protecting
D) as a protection of
E) to protect

here OA is C

now, what is your doubt ?