Tidal power is harnessed by

building a dam across a swiftly flowing river

bombarding uranium nuclei with neutrons

building a dam across the mouth of a bay

tapping into underground steam reservoirs

Building a dam across the mouth of a bay.

Explanation:

Tidal power is harnessed by building a dam (barrage) across the mouth of a bay or tidal turbines. Tidal energy is produced by the use tidal generators, these are placed underwater with high tidal movements. The tide causes water level to rise and trapped in a particular area passes through the dam.

Underwater turbines are designed to capture the kinetic motion of an ocean tides, which produces electricity. Tidal energy has potential to reserve electricity for future use.