The correct answer is A) descending the river. The key to understanding this question is the concept of "left bank" in relation to the direction of travel on a river. The "left bank" refers to the left side of the river when facing downstream, or descending the river. Therefore, a lighted aid to navigation on the left bank would be visible to a vessel traveling downstream, or descending the river. The other answer choices are incorrect because: B) ascending the river would mean the light would be on the right bank, C) crossing the river would mean the light could be seen from either side, and D) proceeding from seaward would mean the light would be on the right bank when traveling upstream.