Final answer:
The king described as old, gracious, and gentle in Shakespeare's 'Macbeth' is King Duncan, the ruler of Scotland, who is contrasted with the ambitious Macbeth.
Step-by-step explanation:
The character in William Shakespeare's play "Macbeth" who is described as the king of Scotland, an old, gracious, pious, and gentle man who resembles Lady Macbeth's father in his sleep is D) Duncan. King Duncan is the benevolent ruler whose murder by Macbeth sets off the tragic events of the play. His characteristics contrast sharply with the ambitious and violent nature of Macbeth. Duncan's murder is pivotal, as it marks Macbeth's first major step on the path to tyranny, instigated by the prophecy of the witches and the goading of his wife.