To determine why a child below seven cannot be brought before the Juvenile Justice Board:
1. Under Section 82 of the Indian Penal Code, 1860, a child below the age of seven is deemed incapable of committing a crime due to lack of maturity and understanding.
2. The Juvenile Justice (Care and Protection of Children) Act, 2015, aligns with this by exempting children below seven from criminal liability.
3. This exemption prevents their prosecution, making it unnecessary to bring them before the Juvenile Justice Board.
Therefore, the correct answer is (3) The child is exempted from criminal liability.