If f(x1)+f(x2)=f(x1+x2) for all real numbers x1 and x2, which of the following could define f?
a) f(x)=x+1
b)f(x)=2x
c)f(x)=1/x
d)f(x)=e^x
e)f(x)=x^2

i have no idea how to go about this, if someone can explain it to me.

Prism m and pyramid n have the same base area and the same height. cylinder p and prism q have the same height and the same base perimeter. cone z has the same base area as cylinder y, but its height is three times the height of cylinder y. the figures and have the same volume.