The correct option is A Dihydrogen oxide
In green plants, dihydrogen oxide also known as water molecules act as the hydrogen donors reducing carbon dioxide to carbohydrates and at the same time releasing oxygen as the by-product during this process.
In purple and green sulfur bacteria, hydrogen sulphide is used as the reducing agent which gets oxidised to form elemental sulfur.
Hydrogen chloride and hydrogen flouride are not used as reducing agents in the process of photosynthesis.