My take is C.
stem 1: n^3(1-x^2)<0
i.e. either n^3<0 or (1-x^2)<0. Not sufficient.
stem 2: x^2-1<0 = x^2<1. Not sufficient.
Now lets combine both the statements:-
According to stem 2: x^2 is less than 1 which means 1-x^2 is +ve. Therefore n^3 should be -ve [according to stem 1]. Thus both the statements taken together are sufficient to ans the question.
Lemme know if i am missing somehting in my approach.
The question is wether X is negative or not. I think you thought the question was concerning N.
X can be -ive or +ive, even in your example
x^2 is less than 1