In Banach space theory, what condition ensures that a subspace W is dense in X?

a) f_x^* = 0 implies f = 0 for every f in x^.
b) f_w^ = 0 implies f = 0 for every f in x^.
c) f_x^ = 0 implies f = 0 for every f in w^.
d) f_w^ = 0 implies f = 0 for every f in w^*.