Explanation:

- Option 1: 100-years-old man

- Age is not a relevant factor for exemption from criminal liability as per Indian law. Elderly people are liable unless proven insane.

- Option 2: 6-years-old boy

- *Correct Answer*: As per Section 82 of the Indian Penal Code, nothing is an offence done by a child under 7 years of age. Children below 7 are exempt due to lack of mature understanding.

- Option 3: 12-year-old girl

- Children above 7 can be liable under law, though courts may consider their maturity under Section 83.

- Option 4: 18-year-old girl

- She is fully liable for her acts, being of majority age.