The correct answer is (d). A guardian ad litem is a person appointed by the court to represent the interests of a minor or an incompetent person in a legal proceeding. The guardian ad litem is responsible for investigating the facts of the case and making recommendations to the court on behalf of the person they are representing.
Option (a) is incorrect because a local commissioner is a person appointed by the court to perform certain duties in a particular case. A local commissioner is not responsible for representing the interests of any party in the case.
Option (b) is incorrect because a court is not a person. A court is an institution that has the power to decide legal disputes.
Option (c) is incorrect because a person defending a suit on behalf of an insane person is not necessarily a guardian ad litem. A guardian ad litem is only appointed by the court if the person they are representing is unable to represent themselves.