The correct answer is option 3, brain. Biological death refers to the complete and irreversible cessation of brain function. When the brain stops functioning, it means that the person has reached the point of no return and is considered clinically dead. The brain is responsible for controlling all bodily functions, including breathing and heartbeat. Therefore, once the brain dies, the body no longer receives essential signals to sustain life.

Options 1, 2, and 4 - kidneys, heart, and lungs - are vital organs, but the death of any of these organs alone does not necessarily imply the biological death of the entire organism. It is possible for these organs to be transplanted and for the person to continue living with the help of medical interventions.