The correct answer is option 'D', which means that all of the mentioned persons are not competent to contract. Let's discuss each category of persons in detail to understand why they are not competent to contract.

a) Minors:
- A minor is a person who has not reached the age of majority, which is typically 18 years old.
- Minors are generally not considered competent to contract because they lack the necessary maturity and understanding to fully comprehend the terms and consequences of a contract.
- However, there are a few exceptions where contracts entered into by minors may be enforceable, such as contracts for necessities like food, clothing, and shelter.

b) Persons of unsound mind:
- Persons who are of unsound mind, including those suffering from mental illness, mental impairment, or any other mental incapacity, are not competent to contract.
- They may lack the mental capacity to understand the nature and consequences of a contract, rendering their consent invalid.
- Contracts entered into by persons of unsound mind are generally voidable, meaning they can be either affirmed or rejected by the person with the mental incapacity or their legal representative.

c) Persons disqualified by law:
- There are certain categories of persons who are disqualified by law from entering into contracts.
- Examples include insolvent persons, convicted felons, and public servants in certain circumstances.
- The disqualification may be based on specific laws or regulations that restrict their ability to contract due to their status or actions.

In conclusion, all of the mentioned persons (minors, persons of unsound mind, and persons disqualified by law) are not competent to contract. Their lack of legal capacity limits their ability to fully understand and consent to the terms of a contract. It is important to ensure that contracts are entered into by competent parties to maintain fairness and protect the rights of all involved.