How to master Euclidean geometry​

Euclidean geometry is all about shapes, lines, and angles and how they interact with each other. There is a lot of work that must be done in the beginning to learn the language of geometry. Once you have learned the basic postulates and the properties of all the shapes and lines, you can begin to use this information to solve geometry problems. Unfortunately, geometry takes time, but if you put in the effort, you can understand it.

(a) the volume of the cone is 20/3 in3

hence, option a is correct.