In a transverse wave that travels through a medium, the molecules of the medium vibrate
a. at right angles to the direction in which the wave travels.
b. at twice the velocity of the molecules in a longitudinal wave.
c. with smaller amplitudes than the molecules in a longitudinal wave.
d. in the direction in which the wave travels

So we wan't to know how the molecules of the medium vibrate when a transverse wave propagates trough that medium. A transverse wave is a wave whos oscilations are perpendicular to the direction of it's propagation. So the molecules in a medium will vibrate perpendicular to the direction of propagation of the wave. So if the wave is moving in the x-direction, the molecules will vibrate in the y-z plane. The correct answer is A. at right angles to the direction in which the wave propagates.