Answer: The correct option is (C) 40 ft.
Step-by-step explanation: Given that a triangle has an area of 100 square feet and a height of 5 feet.
We are to find the length of the base of the triangle.
We know that
the AREA of a triangle with base length 'b' units and height 'h' units is given by

In the given triangle,
Area, A = 100 sq. ft, base length, b = 5 ft and height, h = ?
Therefore, we have

Thus, the length of the base of triangle is 40 feet.
Option (C) is correct.