Question

The boy----the red shirt is my cousin

Answer 1

The boy in the red shirt is my cousin.

I hope its help for you!

Answer 2

This blank can be filled with the word 'in'.

So, correct sentence will be:-

The boy in the red shirt is my cousin.

There are so many other prepositions. But those prepositions can not be used here because it will not give the correct sense of the sentence.

We can define proposition as:

Prepositions are those words which are used before a noun or pronoun and denote the relation between the noun/pronoun and other parts of sentences.