On a power-driven vessel of 20 meters or more in length carrying two masthead lights, the after masthead light must be placed at least how much higher than the forward masthead light?

Why B is correct

33 CFR §84.02(a)(ii) requires that when two masthead lights are carried on a vessel of 20 meters or more in length, the after masthead light shall be at least 2 meters vertically higher than the forward one. The 2.5-meter figure applies to the minimum height above the gunwale for a masthead light on a vessel of 12 to less than 20 meters.