As westerly winds blow over a north-south oriented mountain range, the airflow is deflected in such a way that a trough forms on the side of the mountain

The correct answer is - east.
When the westerly winds flow into a north-south mountain range they will hit the west side of the mountain range, since they are a wind that comes from the west and moves to the east, while the east side of the mountain will get a downward wind on it's slops, that'll be dry and also much weaker and less intense.

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).