Explanation:

- In India, the Constitution divides subjects into three lists: the Union List, State List, and Concurrent List.

- Option 2: Concurrent List - Subjects in this list are common to both the Union and State Governments. Both can legislate on these matters, but in case of conflict, Union law prevails.

- Option 1: Mutual List - There's no official "Mutual List" in the Constitution. This option is incorrect.

- Option 3: Either (a) or (b) - Since "Mutual List" is not a correct term, this option is also incorrect.

- Option 4: Neither (a) nor (b) - Incorrect, as the Concurrent List is indeed common to both governments.