Answer:
A. 12
Step-by-step explanation:
Selecting B, C, or D would create a difference of perfect squares since each of these numbers are perfect squares. Additional factoring would then be needed.
[tex]x^2-36 = (x+6)(x-6)\\x^2-49 = (x+7)(x-7)\\x^2-81=(x+9)(x-9)[/tex]
But selecting 12 gives [tex]x^2-12[/tex] which cannot be factored. It is factored completely.
