If A is a proper subset of B, then A ∪ B

Given, A is a proper subset of B

Let us consider two set as A is a proper subset of B

A = {9, 8, 7, 6, 5, 4} B = {1, 2, 3, 4, 5, 6, 7, 8, 9}

A ∪ B means we must combinedly write all the elements of both the sets.

A ∪ B = {9, 8, 7, 6, 5, 4} ∪ {1, 2, 3, 4, 5, 6, 7, 8, 9}

= {1, 2, 3, 4, 5, 6, 7, 8, 9}

The resultant set is same as B

Option C is the correct answer.