b
(b) $(1 + ax)\,[(1 + {b_1}x)\,(1 + {c_2}x) - (1 + {b_2}x)\,(1 + {c_1}x)]$
+ $(1 + bx)[(1 + {c_1}x)(1 + {a_2}x) - (1 + {a_1}x)\,(1 + {c_2}x)]$
+ $(1 + cx)\,[(1 + {a_1}x)\,(1 + {b_2}x) - (1 + {b_1}x)\,(1 + {a_2}x)]$
= ${A_0} + {A_1}x + {A_2}{x^2} + {A_3}{x^3}$
After solving, the coefficient of $x$ is $0.$