Many constitutional monarchies started out as:
oligarchies
absolute monarchies
dictatorships
democracies

The correct answer to the question: Many constitutional monarchies started out as, would be: Absolute monarchies.

Explanation:

The big difference between an absolute monarchy and a constitutional monarchy, is the limitations placed on the power exercised by a monarch, or head ruler, of a country. In ancient times, this became the norm, especially in Europe, where the absolute power of kings was unquestioned and unchecked by anyone. However, even if kingdoms all over the world started out as absolute monarchies, with the King or Queen being the only law in the land, this changed through time, until these rulers became bound by another law; that of a constitution. This is the case of England, and other such nations, where government went from being solely in the hands of a ruler, to the ruler´s power being chained by constitutions. Today, many of the monarchies only have Kings and Queens as symbols, but they play no part in government.

just don't strip down in public lol