Ans. Hippocampus.
The brain part, mainly associated with long-term memory is hippocampus. Hippocampus is primary memory-storing area of the brain, along with some other regions. Damage to hippocampus, such as due to injury, leads to loss memory before the accident. As Jacob is facing trouble remembering his life before the accident, the part of Jacob's brain that have been injured is hippocampus.