The fossils of multicellular life forms observed in rocks a. are bones that have lithified into rock b. are found mostly in igneous rocks. c. are proof of the young age of the earth. d. are no older than ~600 million years.

d. are no older than 600 million years

Explanation:

Life on Earth appeared relatively early and in very hostile conditions, approximately 3.5 billion years ago, which is only 1 billion years after the formation of Earth. For the first multi-cellular animals to occur though, it took a very long time since life occurred, as the fossils suggest that they only appeared around 600 million years ago, whole 3.9 billion years later than the first uni-cellular lifeforms on Earth. Once the living organisms managed to get to a faze where they are multi-cellular, a real explosion in diversification of the living organisms occurred, and the evolution was rapid. Very soon complex organisms start to appear around the planet, eventually leading to the plants, fish, insects, sinapsids, dinosaurs, mammals, reptilians.