Explanation:

- Option 1: Mere absence of motive of crime cannot in the absence of legal insanity bring the case within Sec. 84.

- This means that simply not having a motive for a crime doesn't mean someone can use legal insanity as a defense.

- Option 2: A person is exonerated from liability for his acts on the ground of unsoundness of mind.

- This refers to someone not being held responsible for their actions if proven mentally unsound during the act. It ties back to legal insanity.

- Option 3: Both (a) and (b).

- Since both statements are accurate and relevant to Sec. 84, this option is correct.

- Option 4: None of the above.

- This is incorrect, as both statements have relevance to Sec. 84.