Final answer:
Stabilize the patient's head.
Step-by-step explanation:
When a young female is involved in a motor vehicle crash and complains of pain to her left eye with a piece of glass impaled in it, the first priority is to stabilize her head to prevent any further injury to her spine. Therefore, you should follow your partner's action and manually stabilize the patient's head. This ensures that the patient's neck is immobilized and any potential spinal cord injury is minimized. The other options mentioned, such as administering 100% supplemental oxygen, stabilizing the impaled glass, or removing the glass from her eye, should be done once the patient's head is properly stabilized.