The correct answer is A. Their protective sleeves.
5.25-inch floppy disks are susceptible to damage from dust, moisture, and heat. Keeping them in their protective sleeves helps to protect them from these elements. The sleeves also help to prevent the disks from being scratched or bent.
Option B is incorrect because plastic bags can trap moisture, which can damage the disks. Option C is incorrect because floppy disks should be stored in an upright position to prevent the magnetic media from being damaged. Option D is incorrect because floppy disks should not be left in the floppy drive when they are not in use. This can cause the disks to become damaged. Option E is incorrect because it is not a safe or effective way to store floppy disks.