答案：C

解析：

Recall that x^2-y^2 can be factored as (x+y)*(x-y). Since x+y = 3 and x-y = 5, their product, x^2-y^2, is (3)*(5), or 15.

Another method of solution is to solve the two equations x+y = 3 and x-y = 5 simultaneously to get x = 4 and y = -1. Then x^2-y^2 is 4^2-(-1)^2 = 16-1 = 15.