Need with number 6 and how did you get it

A rectangular prism is just a shape that looks like a brick.

If you think about a brick for a few seconds, you'll realize
that it has

-- 2 faces of  (length by width)
-- 2 faces of  (length by height)
and
-- 2 faces of  (width by height).

The dimensions of the brick in question 6 are 8ft, 3ft, and 5ft.
So it has

-- 2 faces of  8ft  by  3ft
-- 2 faces of  8ft  by  5ft
and
-- 2 faces of  3ft  by  5ft .

Can you take over and complete it now ?

Ibelieve the answer your looking for is d.