The correct answer is A) ascending the river. When a lighted aid to navigation is shown on the left bank of a river, it means that the light can be seen on the starboard side of a vessel ascending (going upstream) the river. This is because the left bank is on the vessel's starboard side when going upstream. The other options are incorrect because: B) descending the river - the light would be on the port side; C) proceeding towards sea - the left bank reference is specific to a river, not the open sea; D) crossing the river - the light's position on the left bank does not indicate the vessel's direction of travel across the river.