The correct option is B a=0
Given-
f(x)=x3+ax2−7x−6
Option A→ Putting a=0
fx=x3−7x−6
=x3−3x2+3x2−9x+2x−6
=x2(x−3)+3x(x−3)+2(x−3)
=(x−3)(x2+3x+2)
=(x−3)(x2+2x+x+2)
=(x−3)(x+1)(x+2)
So option A is correct option B, C and D are obviously not correct as the factorization of f(x) is complete in 3 simplest factors and no other factors are possible.