Answer: The expression is (C). 6 + x - y.
Step-by-step explanation: We are given to select the expression that is a trinomial of degree 1.
<u>A TRINOMIAL OF DEGREE 1 :</u> It is an algebraic expression with three terms, where the maximum degree of unknown variables in the terms is 1.
(A) The first expression is
Here, we have three terms, so it is a trinomial and the highest power of the unknown variable is 3, not 1. Hence, it is a trinomial of degree 3.
So, option (A) is incorrect.
(B) The second expression is
Here, we have only one term, so it is a monomial and the highest power of the unknown variables is 1 + 2 = 3, not 1. Hence, it is a monomial of degree 3.
So, option (B) is incorrect.
(C) The third expression is
Here, we have three terms, so it is a trinomial and the highest power of the unknown variable is 1. Hence, it is a trinomial of degree 1.
So, option (C) is incorrect.
(D) The fourth expression is
Here, we have two terms, so it is a binomial and the highest power of the unknown variables is 1. Hence, it is a binomial of degree 1.
So, option (D) is incorrect.
Thus, option (C) is correct.