what is equivalent to (3x - 4)^4

a. (3x - 4)(3x - 4)(3x - 4)(3x - 4)
b. (3x + 4)(3x + 4)(3x - 4)(3x - 4)
c. x^4 + 4x^3y + 6x^2y^2 + 4xy^3 + y^4
d. 9x^2 - 24x + 16