What number should be added to the expression x2 + 4x to change it into a perfect square trinomial?

a: 1
b: 2
c: 4
d: 8

Step-by-step explanation:

The given expression is

A perfect square trinomial is a special type of expression that can be factored into the square of binomial expression, as follows

If you use this formula, you would find that if we add the number 4 as a constant term, we would have a perfect square trinomial

Because, it can be factored into

If we develop this square binomial, we would find that its the same than the trinomial we found

Therefore, the right answer is C.