**Explanation:**

A set is a collection of distinct objects, known as elements or members, that have certain properties in common. A set can be finite or infinite, and it can contain any type of objects, including numbers, letters, or even other sets.

In order to determine which of the given options is a set, we need to consider the properties of a set and analyze each option accordingly.

**Option a) The collection of all the months of a year:**

This option represents a collection of all the months of a year. It is a well-defined and finite collection of objects (the months), and each object is distinct. Therefore, this option represents a set.

**Option b) The collection of all interesting novels:**

This option represents a collection of all interesting novels. While it is a collection of objects (novels), the term "interesting" is subjective and can vary from person to person. What one person finds interesting may not be the same for another person. Therefore, this option does not represent a set because it lacks a well-defined and universally agreed-upon criterion for inclusion.

**Option c) The collection of all rich persons of Delhi:**

This option represents a collection of all rich persons of Delhi. Similar to option b, the term "rich" is subjective and can vary in interpretation. The definition of wealth may differ among individuals, and what one person considers rich may not be the same for another person. Therefore, this option does not represent a set because it lacks a well-defined and universally agreed-upon criterion for inclusion.

**Option d) A collection of 10 best writers of India:**

This option represents a collection of 10 best writers of India. However, the term "best" is subjective and can vary in interpretation. Different people may have different opinions on who the best writers are. Therefore, this option does not represent a set because it lacks a well-defined and universally agreed-upon criterion for inclusion.

In conclusion, the only option that represents a set is option a) The collection of all the months of a year. It is a finite collection of distinct objects (the months) that are universally agreed upon.