If Greenwich Mean Time is ahead (of local time) by 12 hours, the place may be (1° = 4 minutes)

The correct choice is: “180° West”

The Earth turns through 360° in 24 hours, so 1° of longitude = 4 minutes of time and 15° = 1 hour. A time difference of 12 hours = 12 × 60 = 720 minutes, and 720 ÷ 4 = 180° of longitude.

Places west of Greenwich see the Sun later, so their local time is behind GMT; places east are ahead. If GMT is ahead of the local time by 12 hours, the place is 180° west of Greenwich: 180° West.

“180° East” would have local time 12 hours ahead of GMT. “90° West” gives a difference of only 6 hours (GMT ahead), and “90° East” gives 6 hours with local time ahead.