The correct answer is - 90° East.
Since the prime meridian starts at 0°, and in total there's 360°, divided into 180 west and 180 east, one quarter of the total number of them will be 90° (360 / 4 = 90). So if we travel eastwards from the prime meridian, and we pass one quarter of the Earth by longitude, we will get to 90° East longitude.