Final answer:
Before going under a raised vehicle, the most crucial safety check is to raise the vehicle slightly and shake it to ensure it is stable and securely supported.
Step-by-step explanation:
The most important safety check that you can perform before going under a raised vehicle is to raise the vehicle slightly off the ground and shake to confirm stability. This action helps ensure that the vehicle is properly supported and will not accidentally fall, potentially causing injury. While checking the oil in the reservoir and rechecking the function of the swing arm restraints are also important, the immediate physical confirmation of stability is paramount to safety. Hence, the correct answer is: a) Raise the vehicle slightly off the ground and shake to confirm stability.