Assertion: Rayon is a semisynthetic polymer whose properties are better than natural cotton.
Reason: Mechanical and aesthetic properties of cellulose can be improved by acetylation.

We begin by carefully examining the assertion. Rayon is produced starting with the natural polymer cellulose obtained from wood pulp or cotton linters. The cellulose is first converted into a soluble derivative, extruded through fine holes, and then regenerated as continuous filaments. Because part of the material is natural (cellulose) and part is processed in a chemical plant, we call it a semisynthetic polymer. Compared with natural cotton, rayon generally shows higher luster, better drape, and can be engineered for greater tensile strength. All these facts show that the assertion “Rayon is a semisynthetic polymer whose properties are better than natural cotton” is correct.

Now we consider the reason. Cellulose contains three reactive hydroxyl (-OH) groups per glucose unit. When cellulose is treated with acetic anhydride in the presence of an acid catalyst, the -OH groups are converted into acetate ester groups (-OCOCH3). This reaction is called acetylation. The general chemical change can be written as:

$$\text{Cellulose - OH} \;+\; (CH_3CO)_2O \;\rightarrow\; \text{Cellulose - OCOCH}_3 \;+\; CH_3COOH$$

Introducing acetate groups reduces intermolecular hydrogen bonding and imparts thermoplasticity, improved crease recovery, and a silk-like luster. These are all mechanical and aesthetic improvements. Therefore the statement “Mechanical and aesthetic properties of cellulose can be improved by acetylation” is true, so the reason is correct.

Next we must decide whether the reason actually explains the assertion. Rayon covers both viscose rayon (regenerated cellulose) and acetate rayon (cellulose acetate). The very improvements in strength, luster, and feel that make rayon superior to cotton arise largely because chemical modifications such as acetylation (or xanthation in the viscose process) break the rigid hydrogen-bond network of native cellulose and allow the fibers to be spun with controlled orientation and finer diameter. Thus, by pointing specifically to acetylation of cellulose, the reason supplies a direct chemical basis for why a regenerated cellulose fiber like rayon can outperform natural cotton.

Both sentences are individually correct, and the chemical improvement cited in the reason is indeed one of the key causes behind the better properties mentioned in the assertion. Consequently, the reason is the correct explanation of the assertion.

Hence, the correct answer is Option B.