Which value must be added to the expression x^2 + 16x to make it perfect-square trinomial?

A-8

B-32

C-64

D-256

Edit: it's not A

X^2 + 16x + ?

a^2 + 2ab + b

(a + b) ^2 which is a perfect square.

To find b to determine the last value, you divide 16x by 2x.

so:

b = 8

the last value would be 8^2 which is 64.

Hence answer is C - 64