ANS:B - geoid surface

The geoid is an imaginary sea level surface that has zero elevation and that undulates (has a wavy surface) over all of the earth.

Mean sea level (MSL) is the average level of the surface of various oceans from which heights such as elevations may be measured. The Mean Sea Level (MSL) adopted by a survey of India for reference is located at Mumbai High. The average level of the ocean's surface, calculated as the arithmetical mean of hourly tide levels taken over an extended period and used as the standard for determining terrestrial and atmospheric elevations and ocean depths.

Geoid surface is the surface of zero elevation. The surface of zero elevation around the earth, which is slightly irregular and curved is known as (Mean sea level).