Assertion
Cotton is a polymer
Reason
A polymer is a very big molecule made of many small repeating units of compounds called monomers
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct but reason is not the correct explanation for assertion
C. Assertion is correct but reason is incorrect
D. Both assertion and reason are incorrect

Hint: In the word polymer, poly- means many, which means that the polymers are made up of repeating units of monomers which are small compounds having two or more than two functions.

The polymers could be characterised into two types, namely natural and synthetic polymers. Protein is a very common example of natural polymer, whereas polythene is an example of synthetic polymer.

Complete step by step answer:
Polymers are repeating units of monomers, for instance, we know that polythene is made up of repeating units of ethene. However there are certain criteria which a molecule should fulfil in order to become a monomer, every small compound cannot become monomer. In order to become a monomer, the compound should have functionality two or more than two, which means that the compound should be able to form new bonds from two or more sites. A compound which is bifunctional can only form straight chain polymeric compounds like polythene or polypropene, while in case of compounds with more than two functionality, the polymer could form branched chains.
Now if we consider the assertion given in the question, cotton is a natural polymer which has cellulose content. Cellulose is nothing but a long chain of unbranched polymers, hence is linear. The monomers of the cotton are glucose units which are together known as cellobiose. So we can say that the assertion given in the question is true, so we can eliminate the option D totally, which says both are incorrect. So, now we are left with three options.
Now, if we consider the reason given in the question, we know from what we have discussed earlier, that the polymers is a huge molecule which could also be termed as macromolecule, with repeating units of monomers, so the reason is also true.
So if we correlate both of them, cotton is a type of polymer because it is a large molecule made up of repeating units of monomers. Hence the correct option is B, Since both assertion and reason are correct, but reason is not the correct explanation for the assertion. Cotton is a polymer not just because polymers are made up of monomers. Also cotton is a natural polymer

So, the correct answer is Option B .

Note: Cotton is a natural polymer which is made up of cellulose, and polymers are a class of compounds which are made up of repeating units of monomers having more than one reactive sites also termed as functionality of a monomer.

The monomers with only two reactive sides, also termed as bifunctional, can form polymers of straight chains only, which doesn’t have any branching in them.