A solid frustum of a cone is of height 8 cm. If the radii of its lower and upper ends are 3 cm and 9 cm respectively, then its slant height is:

Correct option is C

Given:

Height  h  = 8 cm

Radius of lower base R  = 9 cm
Radius of upper base r = 3 cm

Formula Used:

​$$\text{Slant height} (l) = \sqrt{h^2 + (R - r)^2}$$​
where:
h  is the height of the frustum,
R  is the radius of the lower base,
r  is the radius of the upper base.
Solution:

l = $$\sqrt{8^2 + (9 - 3)^2}$$​​

l = $$\sqrt{64 + 36} = \sqrt{100}$$​

l = 10 cm
Option (C) is right.