The correct answer is: A. Agreement in restraint of marriage.
An agreement in restraint of marriage is a contract that prohibits one or both parties from marrying another person. Such agreements are void because they are against public policy. The law encourages marriage and does not want to allow people to enter into contracts that would prevent them from getting married.
Option B is not a void agreement. An agreement which is expressed in writing and registered under the law for the time being in force for the registration of documents, and is made on account of natural love and affection between parties standing in a near relation to each other is a valid contract.
Option C is not a void agreement. An agreement to compensate, wholly or in part, a person who has already voluntarily done something for the promisor, or something which the promisor was legally compellable to do is a valid contract.
Option D is not a void agreement. An agreement made inwriting and signed by the person to be charged therewith or by his agent generally or specially authorized in that behalf, to pay wholly or in part debt of which the creditor might have enforced payment but for the law for the limitation of suits is a valid contract.
In conclusion, the only void agreement among the choices is A. Agreement in restraint of marriage.