Which function has only one x-intercept at (−6, 0)?

Which function has only one x-intercept at (−6, 0)?

a. f(x) = x(x − 6)
b. f(x) = (x − 6)(x − 6)
c. f(x) = (x 6)(x − 6)
d. f(x) = (x 6)(x 6)