Which command is used to see the sub-directory structure of drive?

Answer: Option A

The Tree command is used to display the directory structure of a drive or path in a graphical representation.

List is not a command used in DOS to display sub-directory structures.

Subdir is not a recognized command in DOS for displaying sub-directory structures.

Subtree is also not a valid DOS command for displaying directory structures.

Therefore, the correct answer is Option A: Tree.