The correct option is C x2+2x+3
The expressions 2x + 3y and x2+2y, both have two variables, x and y.
The expression x2+2x+3 has one variable, which is x. Hence, it is an expression in one variable. The degree of this expression, or the highest exponent on x, is 2.
The expression 5 is a constant. It has no variables.