Photovoltic cells are

Photovoltaic cells are solar cells, so the answer is A. The term describes the photovoltaic effect: when light photons strike a semiconductor junction, usually silicon, they free electrons and produce a voltage and current. Edmond Becquerel observed this in 1839, and Bell Labs made the first practical silicon cell in 1954.

• B (Thermal cells): Thermal devices convert heat to energy, as in solar thermal collectors or thermocouples. They do not work through the photovoltaic effect, so the term does not apply.
• C (Sulphur cells): Sulphur-based cells, such as sodium-sulphur batteries, are electrochemical storage devices. They generate no power from light.
• D (Molar cells): 'Molar' is a unit of concentration in chemistry. No such category of cell exists.

For Mains, link this to GS-III energy security and the National Solar Mission under the JNNSM (2010). Solar PV capacity has grown sharply since 2016, so the figures in your notes should be checked against current data.

Trap: Aspirants sometimes confuse solar PV with solar thermal technology. Remember that PV converts light directly into electricity, while solar thermal uses heat.