The situation shown consists of a picture with the sun emitting light energy; and a plant both absorbing carbon dioxide and emitting oxygen. The missing reactant in order for photosynthesis occur is water. Plants absorb water from the soil (through roots) and absorbs ligth energy from the sun (through leaves). The energy absorbed is used to split water molecules into H2 and O2. Then H2 combines with CO2 to form glucose. The answer is the option A) water.
