Diamonds are formed when magma containing small amounts of carbon is deeply buried and

a)

is deeply buried and slowly cooled under high pressure

b)

rises near earth's surface and is slowly cooled under low pressure

c)

is deeply buried and quickly cooled under high pressure

d)

is deeply buried and slowly cooled under low pressure

A)  is deeply buried and slowly cooled under high pressure

Explanation:

Diamond Formation in Earth's Mantle. Geologists believe that the diamonds in all of Earth's commercial diamond deposits were formed in the mantle and delivered to the surface by deep-source volcanic eruptions. The formation of natural diamonds requires very high temperatures and pressures.

Diamonds from Deep-Source Eruptions: Most commercial diamond deposits are thought to have formed when a deep-source volcanic eruption delivered diamonds to the surface. In these eruptions, magma travels rapidly from deep within the mantle, often passing through a diamond stability zone on its route to the surface.

Diamond-bearing kimberlite pipes are only found in ancient areas of the continents. But diamonds have also been found far from the pipes. This is because volcanic rock erodes and is washed away over time. Rivers, streams and glaciers can carry diamonds far from the original volcanoes.