What form of government is ruled by a king, queen, emperor, or empress?

monarchy

A monarchy has a king, queen, emperor or empress. The ruling position can be passed on to the ruler's heirs. In some traditional monarchies, the monarch has absolute power. But a constitutional monarchy, like the UK, also has a democratic government that limits the monarch's control

d is the right answer

that was the type of government the spanish colonists were used to.