Question

what is a prism .?????

Answer 1

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. At least two of the flat surfaces must have an angle between them.

Answer 2

A cross section is the shape made by cutting straight across an object.

The cross section of this object is a triangle ...

.. it has the same cross section all along its length ...

... so it's a triangular prism.