What marks the beginning of the Precambrian Era?

The beginning of the Precambrian Era is marked by the formation of the Sun and Earth. This era spans a vast amount of time, covering nearly 90% of the Earth's history, and it encompasses the formation of our planet, which occurred about 4.6 billion years ago, alongside the formation of the solar system, including the Sun. During this time, the Earth underwent significant geological, atmospheric, and biological changes, although multicellular life did not appear until later.

The other choices represent significant events that occur after the Precambrian Era. For example, the extinction of dinosaurs occurred at the end of the Mesozoic Era, well after the Precambrian. The creation of plants and animals emerged in later geological periods, such as the Cambrian Explosion, when diverse life forms first appeared. The rise of mammals also took place after the extinction of the dinosaurs, specifically during the Cenozoic Era, which succeeded the Mesozoic. Thus, the formation of the Sun and Earth is the foundational event that correctly signifies the start of the Precambrian Era.