The lateral faces of a regular triangular prism are

isosceles triangles
equilateral triangles
parallelograms
rectangles

The lateral face of a prism is it's side, so it would be rectangle, D.
 
"lateral, of, at, toward, or from the side or sides."

The lateral side of a triangular prism would be a rectangle, like in this picture. 
The picture shows two quadrilateral triangles as the faces of the prism, and three rectangles as the lateral sides.