Which one of the following is essential for photolysis of water?

The correct option is Manganese.
Photolysis of water is the process of breakdown of a water molecule into hydrogen and oxygen under the influence of light during the light reaction of photosynthesis.
2H₂O → 4H⁺ + 4e⁻ + O₂
Photolysis of water requires light energy and OEC (oxygen evolving complex) and an electron carrier.
OEC has four Mn ions. Light energy brings about changes in Mn which help in removing the electron from the OH⁻ component of water forming oxygen.
Hence Manganese (Mn) is essential for photolysis of water.
So, the correct answer is 'Manganese'.