The correct option is A 3x(x+1)
21x(x2−1)=3× 7× x× (x+1)(x−1).
3x3(x+1)=3× x3× (x+1).
Hence, the common irreducible factors between 21x(x2−1) and 3x3(x+1) are 3,x and (x+1).
The least exponents of these factors are respectively 1,1 and 1.
Hence, the HCF = 31× x1× (x+1)1=3x(x+1).