In cyclic photophosphorylation, the process of converting light energy into chemical energy occurs in the thylakoid membranes of chloroplasts. This process involves the synthesis of ATP, which is an energy-rich molecule that can be used by cells for various metabolic reactions.

Cyclic photophosphorylation involves a cyclic flow of electrons through the electron transport chain. It occurs when there is an excess of ATP relative to NADPH in the chloroplasts, which is often the case under certain light conditions.

Here is a detailed explanation of the process and why the correct answer is option 'B' (ATP):

1. Light absorption:
- Light energy is absorbed by chlorophyll pigments in the photosystems located in the thylakoid membranes.
- This energy excites electrons in the chlorophyll molecules, raising them to a higher energy level.

2. Electron transport chain:
- Excited electrons are passed from one molecule to another in a series of redox reactions, forming an electron transport chain.
- During cyclic photophosphorylation, these electrons are passed from the primary electron acceptor of photosystem I (PSI) to a protein complex called ferredoxin (Fd).

3. ATP synthesis:
- As the electrons move through the electron transport chain, they release energy that is used to pump protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient.
- This proton gradient drives the synthesis of ATP through a process called chemiosmosis.
- ATP synthase, located in the thylakoid membrane, uses the flow of protons down their concentration gradient to catalyze the conversion of ADP and inorganic phosphate (Pi) into ATP.

4. Electron flow:
- After passing through ferredoxin, the electrons are transferred back to the cytochrome complex, where they can re-enter the electron transport chain of photosystem I.
- This cyclic flow of electrons allows for the generation of additional ATP molecules.
- Importantly, there is no production of NADPH during cyclic photophosphorylation.

In conclusion, in cyclic photophosphorylation, the primary product formed is ATP, which is synthesized through the movement of electrons in the electron transport chain and the generation of a proton gradient. This process does not involve the production of NADPH or the release of oxygen (O2).