Final answer:
The safety pin of an MK1 Illumination grenade can be replaced after being pulled, provided the grenade has not detonated, and this is done by trained individuals only. Other components like the trigger mechanism, fuse, and lever cannot be simply replaced once activated.
Step-by-step explanation:
The answer to the question, "What can be replaced on the MK1 Illumination grenade after being pulled?" is b) Safety pin. Once the safety pin of an MK1 Illumination grenade has been pulled, and provided that the grenade has not been detonated, the safety pin can typically be reinserted, effectively 'safeing' the grenade. However, this action should only be performed by individuals who are properly trained and thoroughly understand the workings of the grenade. It is crucial to note that handling live grenades is extremely dangerous and should be done with great caution. The other components listed, such as the trigger mechanism, fuse, and lever, are integral parts of the grenade's firing mechanism and once activated or removed, they cannot be simply replaced or reset without the proper equipment, expertise, and safety measures.