The radius $\ ( in\ cm)$ of the largest right circular cone that can be cut out from a cube of edge 4.2 cm is

$( A) \ 4.2$ cm
$( B) \ 2.1$ cm
$( C) \ 8.4$ cm
$( D) \ 1.05$ cm

Given: A cube of edge $4.2\ cm$.

To do: To find out the radius of the largest right circular cone to be cut out from the cube.

The largest right circular cone that can be cut out from the given cube, its diameter would be equal to side of the cube. And height of the cone would be also equal to the side of the given cube.

$\therefore$ Diameter of the cone $=$ side of the cube $=4.2\ cm$

Radius $=\frac{diameter}{2} =\frac{4.2}{2} =2.1\ cm$

$\therefore$ Option $( B)$ is correct.