Which value must be added to the expression x squared plus 16 x to make it a perfect-square trinomial?

For an expression x² + bx to become a perfect-square trinomial, add the square of half the coefficient of x. Here, half of 16 is 8, and 8² = 64. Thus x² + 16x + 64 = (x + 8)², so the correct choice is 64.

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