The first protists probably appeared around a. 1.5 million years ago b. 170 million years ago c. 1.5 billion years ago d. 5 billion years ago

Option (C)

Explanation:

Protists are the kingdom that includes unicellular organisms. These organisms are eukaryotic in nature which means that it has a well-defined nucleus. They may be present in colonies or biofilm. But they re self-sufficient. Since the time of formation of these organisms, they have constantly undergone evolution forming a more complex type of organisms.  

The first protists appeared during the proterozoic age at about 1.8-1.5 billion years back.

Hence the most appropriate answer is 1.5 billion years ago i.e option (C)

what's the question ?

the answer is "a.) well–developed vascular system."

explanation: