Which of the following expressions are perfect square trinomials?

Remember that a perfect square trinomial can be factored into the form (a+b)^2
or (a-b)^2
Examples:
(x+2)(x+2) is a perfect sq trinomial --> x^2+4x+4
(x-3)(x-3) is a perfect sq trinomial --> x^2-6x+9
(x+2)(x-3) is not a perfect square trinomial because its not in the form (a+b)^2 or (a-b)^2

Now to answer your question,

for the first one, x^2-16x-64, you cannot factor it so it is not a perfect square trinomial

for the second one, 4x^2 + 12x + 9, you can factor that into (2x+3)(2x+3) = (2x+3)^2 so this is a perfect square trinomial

for the third one, x^2+20x+100 can be factored into (x+10)(x+10) so this is also a perfect square trinomial

for the fourth one, x^2+4x+16 cannot be factored so this is not a perfect square trinomial

Therefore, your answer is choices 2 and 3