The energy potential from the sea (tidal energy, wave energy and ocean thermal energy) is quite large, but efficient commercial exploitation is difficult.

OCTEC stands for ocean thermal energy conversion power plant. It is a device used to harness tidal energy to produce electricity.

Tide  are the rise and fall of sea levels caused by the combined effects of gravitational forces exerted by the Moon, Sun and rotation of the Earth. The effect of moon is most prominent among all the factors. Rise of ocean level is called high tide and fall of ocean level is called low tide.

$71 \%$ of the Earth's surface is covered by oceans. Hence, most of the solar energy is absorbed by the oceans. This energy can be harnessed using OCTEC. Hence, ocean thermal energy has great potential.

Energy from the sea can be classified into three types:

Temperature gradient of the water with deep ocean being cold and tropical surface water being warm due to the Sun's heat is the ocean thermal energy.

Dams or reservoirs are usually build near the narrow opening of the seashores to harness the energy of the waves during high tide in generation of electricity.

Waves are made to fall from a height on to the rotor of a turbine or dynamo to make them rotate. This mechanical energy is then converted to electrical energy.

The two different ways of harnessing energy from ocean is Tidal wave and Ocean Thermal Energy Conversion (OTEC).
Answer (A) Tidal Wave, OTEC

Energy of flowing water

The hot water in the ocean is used to evaporate liquids like ammonia or CFC. This vapour is used to rotate the turbines of the generator and the vapour is again cooled down to a liquid by using the cold water from the ocean depth. This cooled liquid ammonia or CFC is again vapourised and the above process continues.

Renewable energy obtained from the ocean is together termed as ocean energy. Thus, ocean energy can be classified into various categories such as ocean thermal energy, tidal energy and wave energy.

Tidal -Because of  gravitation pull of Moon, tides in sea are seen.Tidal energy is a form of hydropower which utilizes energy from tides to produce electricity.
Wave - Waves are good source of energy. Waves from the sea are used to run the turbines to generate electricity.
Ocean thermal energy - Water at sea surface is hot during daytime and at lower level is cold. This temperature difference is used to produce energy (electric).

In wave energy, kinetic energy possessed by huge waves is trapped to generate electricity with the help of a machine called wave energy convertor. 

Difference in sea-levels due to tides during different times of day gives us tidal energy. Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity.

Wave energy would be a viable proposition only where waves are very strong.