Explanation:

- Option 1: Conspiracy in S.120A is mere agreement to commit an offence.

- This is correct. Under Section 120A of IPC, mere agreement between parties to commit an offense is the essence of conspiracy.

- Option 2: Instigation by engaging in conspiracy requires an act or illegal omission.

- This is also correct. Instigation involves encouraging or provoking a person to commit an offense, which often goes beyond mere agreement and involves acts or omissions.

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

- This option includes both of the above, which are both correct.

- Option 4: None of the above.

- This is incorrect since both statements in Options 1 and 2 are correct.

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