Photovoltaic solar cells are:

a) all of the above
b) cheap to produce, but contribute to atmospheric pollution, including CO2
c) wafers of crystalline silicon treated with metals that absorb solar radiation and generate electricity
d) single-celled organisms that are used to collect solar radiation, which they turn into heat.

c) wafers of crystalline silicon treated with metals that absorb solar radiation and generate electricity

Explanation:

Photovoltaic solar cells are used in solar panels. Most of the photovoltaic cells are made from crystalline silicon.

It has a property using which it converts the light (solar radiation) falling on it to electrical energy. This process occurs due to the photovoltaic effect.

It is a way generating energy without consuming fossil fuels. Thus it reduces the carbon emission and thus plays a major role in environment protection.

d. air pressure decreases as altitude increases.