Answer:

The right answer is A “the death of his father”. One of the main themes of Hamlet is that “Death Conquers all”, meaning that death plays an important role in the play. The queen says to Claudius, her ex husband’s brother and his following husband after becoming a widow, "I doubt it is no other but the main: His father’s death and our o'erhasty marriage". With this line we can see how she clearly thinks her son’s madness is because of the death of his father.