Final answer:
The bottom sheets must be tight and wrinkle-free to ensure comfort, hygiene, and a neat appearance. They are in direct contact with the sleeper's skin, making it particularly important for them to be smooth.
Step-by-step explanation:
The linens that must be tight and wrinkle-free are A. Bottom sheets. The importance of having tight and wrinkle-free bottom sheets is not only for aesthetic purposes but also for comfort and hygiene.
A smoothly laid bottom sheet prevents the accumulation of dust mites and other allergens, which can be exacerbated by wrinkles or folds in the fabric. While it's preferred that other bed linens such as blankets, bedspreads, and pillowcases are also wrinkle-free for a neat appearance, it is the bottom sheet that directly affects the sleeper's skin and therefore has a more critical requirement for smoothness.