Answer: The correct option is (C). 10 feet.
Step-by-step explanation: We are given to find the slant height of a square pyramid that has a surface area of 189 square feet and a side length of 7 feet.
We know that the surface area of a square pyramid with base edge 'a' units and height 'h' units is given by

A square pyramid is shown in the attached figure.
In the given square pyramid, we have
length of the base edge, a = 7 feet,
Surface area, S.A. = 189 sq. ft.
If 'h' is the height of the pyramid, then we have
.
So, if 'l' is the slant height of the pyramid, then

Thus, the slant height of the square pyramid is 10 feet.
Option (C) is correct.