Explanation:

- According to Section 320 of the Indian Penal Code (IPC), ‘grievous hurt’ includes any hurt that causes someone to be in severe bodily pain or makes him/her unable to follow ordinary pursuits for a certain period.

- The law specifically states that if the person is unable to do their regular work for twenty days, the hurt is considered ‘grievous’.

- Let’s look at your options:

- Option 1: Fifteen days (?) – this is not correct, it's too short according to the law.

- Option 2: Twenty days () – this is correct. The law mentions twenty days.

- Option 3: Twenty five days (?) – this is longer than the required period.

- Option 4: Thirty days (?) – this is also incorrect, too long.

So, the correct answer is Option 2: Twenty days ().