(6-x)/(x^2+2x-3)
--------------------
(x^2-4x-12)/(x^2+4x+3)
(6-x)/(x+3)(x-1)
-----------------------------
(x-6)(x+2) / (x+3)(x+1)
(6-x)/((x+3)(x-1)) * ((x+3)(x+1))/((x-6)(x+2))
(6-x)(x+3)(x+1)
-------------------------
(x-6)(x+2)(x+3)(x-1)
-(x+1)/((x+2)(x-1))
-(x+1)/(x^2+x-2)
From the final result we must exclude 1 and -2
However, (C) is not the answer, since in the original expression, -3 and 6 must also be excluded, since they produced zero in fraction denominators.
So, (A) is the answer.