The correct answer is A. Pooling.
Pooling is the process of collecting premiums from numerous individuals to compensate the few who may suffer losses. This is the basic principle of insurance, and it allows insurers to spread risk and provide coverage to a large number of people.
Option B, contract, is not the correct answer because a contract is simply an agreement between two or more parties. It does not necessarily involve pooling of risk.
Option C, guarantee, is not the correct answer because a guarantee is a promise to pay a certain amount of money in the event of a loss. It does not necessarily involve pooling of risk.
Option D, IRDA, is not the correct answer because IRDA is the Insurance Regulatory and Development Authority of India. It is a government body that regulates the insurance industry in India. It does not have anything to do with pooling of risk.