With a head injury, the victim is laid in a semi-recumbent position, head and torso raised, so that the pressure in the skull does not rise. Raising the legs would send blood to the head, and a horizontal position does not help the blood drain away from the head.
With a head injury, it is very important to reduce or prevent a rise in pressure inside the skull. Laying the victim in a semi-recumbent position with the head and torso raised helps blood drain toward the lower parts of the body, effectively lowering the intracranial pressure.
Option 1. Raising the legs increases blood flow to the head, which can dangerously raise the intracranial pressure.
Option 3. A fully horizontal position does not help gravity drain blood toward the lower parts of the body.