Which of the following expressions represents a sum?

The expression that represents a sum is the one that uses the addition operation, which combines quantities by increasing one by the amount of the other. In this case, the expression "x + y" clearly indicates that you are adding the values of x and y together, resulting in their total or sum.

This distinguishes it from other operations such as subtraction, division, or multiplication. Subtraction would indicate a difference, division represents a quotient, and multiplication shows a product. Therefore, "x + y" is the only expression among the listed options that explicitly signifies the act of summation.