Many present-day animal phyla appeared suddenly in fossils formed 535-525 million years ago, during the paleozoic era of the phanerozoic eon. this period is referred to as the cambrian explosion. does this period mark the beginning of animal life?

The correct answer is - No.

The Cambrian explosion is a term used for the big and rapid diversification of the animal species, but it is not the period in which the animal life started. The animal life started in the period between 620 and 550 million years ago, in what is known as the Vendian Period. In the Vendian Period, the first complex animal organisms appeared, started to develop and evolve, though it is not a period where there was a high diversification of the species. The later Cambrian Period provided better living conditions, and that resulted in a so called ''explosion of life'', which resulted in much more new species developing and evolving.

it is either c or d and i know for sure it is not a or b