Which equation shows the reaction of photosynthesis?

Photosynthesis is the light-driven synthesis of glucose from carbon dioxide and water, releasing oxygen as a byproduct. In the light-dependent reactions, water molecules are split (photolysis) to supply the electrons and protons needed to fix carbon dioxide into sugar during the Calvin cycle -- the oxygen released comes from this water, not from carbon dioxide. Because part of the water consumed in the light reactions is regenerated by the Calvin cycle, the complete balanced equation for the overall process must show water appearing on both sides -- as a reactant and, in smaller amount, as a product.

Application:

Applying this to the given options, the complete equation must show twelve water molecules consumed as reactant (supplying the electrons/protons split off to release six O2 molecules) and six water molecules regenerated as product: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O. This is the form given in NCERT Class 11 Biology and the NCERT Class 10 Science Exemplar, and the official UPTET 2016 Paper 2 answer key for this exact question credits this option as correct. A simplified net equation with the water terms combined into a single coefficient is also used informally to summarise the same overall reaction, but it is not the option credited by this exam's official key.

Cross-check / contrast with the other options:

• CO2 + H2O → CH2O + O2 is atom-balanced only as a generic one-carbon-fixed unit; it uses a placeholder formula for the sugar product instead of the specific hexose sugar that photosynthesis is understood to yield, so it is not the form used in the reference textbooks for this reaction.

• 6CO2 + 6H2O → C6H12O6 + 6O2 is the simplified net equation, obtained by cancelling the water that appears on both sides of the full reaction; it is a valid summary of overall inputs and outputs but does not itself display the water-splitting step of the light reactions.

• C6H12O6 + 6O2 → 6CO2 + 6H2O runs in the reverse direction: glucose and oxygen breaking down into carbon dioxide and water is the summary equation of cellular respiration, not photosynthesis.

Hence, 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O is the correct and complete representation of the photosynthesis reaction, per the official PYQ key and NCERT.