What part of the water is affected by a wave in deep water

There is a thing called "wave action". In a submarine, you can feel wave action down to about 100 feet or so. Below 100 feet, the wave action is insignificant. The reason is buoyancy. As a wave passes over you, in effect the water gets deeper, there is more over your head, and alternately, as a trough passes over you, the water gets shallower. This alternating change in depth causes you to bob up and down like a cork. This all changes as the water gets shallower. A wave is really a moving pressure front. As the water gets shallower, the speed increases, which is why a wave "breaks" as it approaches a shore line but is invisible further away. The Tsunami in Japan that traveled across the Pacific was invisible until it reached the shallower water off the Pacific Coast where it did significant damage. What was a few INCHES in the middle of the Pacific became several FEET at the shoreline, all because the water went from very deep to very shallow. I was in submarines in the Navy for 21 years of service. Wave action is a very real thing and being at periscope depth in a hurricane is like a roller coaster as the boat gets thrown around.

wave base is the most affected by a wave in deep water

You would do 1500/4 and the answer is 375