With a suspected spinal injury, the main thing is not to move the spine, so as not to damage the spinal cord. Laying the victim on the side or putting a roll under the neck twists or bends the spine, so the victim is laid on their back in a hard, flat place and not moved unnecessarily.
With a suspected spinal injury, the main goal is to prevent any movement of the spine. A conscious victim should be laid on their back on a hard, flat surface and kept from unnecessary movement. Any bending or twisting of the back or neck can damage the spinal cord and lead to permanent paralysis.
Option 1. Laying the victim on the side twists the spine, which can make the injury worse.
Option 3. Putting a roll under the neck bends the neck section of the spine, which can seriously damage the spinal cord.