Explanation:

The correct answer is option B, "promisor".

Consideration:
Consideration is an essential element in a contract. It refers to something of value given by one party in exchange for something of value given by the other party. It can be in the form of money, goods, services, or a promise to do or not to do something.

The Principle:
In contract law, the consideration must move at the desire of the promisor only. This means that the consideration should be given in response to the promisor's request or desire. It is the promisor who makes an offer or a promise, and the consideration is given by the promisee in response to that offer or promise.

Role of Promisor:
The promisor is the party who makes the offer or promise in a contract. They are the ones who initiate the transaction and express their desire to enter into a contract. The promisor's offer or promise is the basis for the consideration given by the promisee.

Role of Promisee:
The promisee is the party to whom the offer or promise is made. They are the ones who receive the offer or promise and give consideration in response to it. The promisee's consideration is given because of the desire or request of the promisor.

Example:
For example, let's say John offers to sell his car to Mary for $10,000. Mary agrees to buy the car and gives John $10,000 in exchange. Here, John is the promisor who made the offer to sell the car, and Mary is the promisee who gave consideration by paying the agreed amount.

Conclusion:
In conclusion, the consideration in a contract must move at the desire of the promisor. The promisor is the party who makes the offer or promise, and the consideration is given by the promisee in response to that offer or promise. This principle ensures that there is a mutual exchange of value between the parties involved in the contract.