(x+a)(x+b)=x^2+x(a+b)+ab
x^2-x-6=x^2+x(a+b)+ab
so,
you have a+b=-1 and ab=-6
in A , a=-2 and b=3 ⇒ a+b=-2+3=+1 , ab=-2(3)=-6 so A is not the answer
in B, a=-1 and b=-6 ⇒ a+b=-1-6=-7 , ab=-6(-1)=6 so B is not the answer
in C, a=+2 and b=-3 ⇒ a+b=+2-3=-1 , ab=+2(-3)=-6 so C is the answer
in D , a=+1 and b=-6 ⇒ a+b=+1-6=-5 , ab=+1(-6)=-6 so D is not the answer