Answer:
B. (cos 3π/4 + i sin 3π/4)
Step-by-step explanation:
DeMoivre's theorem tells you the cube of a complex number is the cube of its magnitude at 3 times the angle. The given number has a magnitude of 1, so its cube will also. Three times the angle of the given number is ...
3 × π/4 = 3π/4
so the number you're looking for among the answer choices is ...
(cos(3π/4) +i·sin(3π/4)) . . . . matches choice B