According to Encyclopaedic Law Dictionary, where a person unjustly obtains a benefit at the expense of another. In certain cases where money is obtained by mistake or through fraud or for a consideration which has wholly failed, the law implies a promise to repay it. The rule against unjust enrichment is embodied in section 70 of Indian Contract Act, 1872 and founded not upon any contract or tort but upon a third category of law, namely quasi contract or restitution.

The correct answer is option 'A' - Quasi contract.

A quasi contract is a legal concept that is used to prevent unjust enrichment. It is a principle that does not allow a person to retain an unjust benefit at the expense of another. In other words, it is a legal remedy that is used when there is no contractual relationship between the parties involved, but one party has received a benefit at the expense of another party.

A quasi contract is also known as a contract implied in law, because it is not based on the actual intentions or agreements of the parties involved. Instead, it is based on the principle of fairness and preventing unjust enrichment.

Below are some key points to understand the concept of a quasi contract:

1. Definition of a quasi contract: A quasi contract is a legal fiction created by the courts to prevent one party from being unjustly enriched at the expense of another party. It is sometimes called an implied contract because it is not based on an actual agreement between the parties.

2. Elements of a quasi contract: To establish a quasi contract, certain elements must be present. These include:
- One party has received a benefit from another party.
- The receiving party has knowledge of the benefit.
- The receiving party would be unjustly enriched if they were allowed to keep the benefit.
- The circumstances are such that it would be unfair to allow the receiving party to retain the benefit.

3. Purpose of a quasi contract: The purpose of a quasi contract is to prevent one party from profiting at the expense of another party without a valid contractual relationship. It ensures fairness and equity by requiring the party that received the benefit to compensate the other party for the value of that benefit.

4. Examples of quasi contracts: Quasi contracts can arise in various situations, such as:
- A person mistakenly pays for someone else's expenses.
- A person receives goods or services without entering into a formal contract.
- A person benefits from another person's labor or services without an agreement.

5. Remedies in a quasi contract: When a court determines that a quasi contract exists, it may provide remedies to the aggrieved party. These remedies typically involve requiring the party that received the benefit to compensate the other party for the value of the benefit received.

In conclusion, a quasi contract is a legal principle that prevents unjust enrichment. It is used when there is no formal contract between the parties involved, but one party has received a benefit at the expense of another. The purpose of a quasi contract is to ensure fairness and equity by requiring the receiving party to compensate the other party for the value of the benefit received.

Because it follows the concept of "Doctorine of unjust-enrichment"