The term protista refers to:

a) A kingdom within the superkingdom SAR.

b) A kingdom within the superkingdom Excavata.

c) It is a descriptive term or common name for unicellular, colonial, and some multicellular organisms that lack true tissues.

d) A phylum within the Kingdom Metazoa.

The correct answer will be option C.

Explanation:

Protista is a group of Eukaryotic organisms which consists of unicellular organisms.

These protists remain unicellular which live in colonies as well as some group live as a simple multicellular organism. these multicellular organisms lack complexity, unlike true multicellular organisms. Protists exhibit the characters of plants, animals and fungi but lack true tissues like seaweed kelp.

Thus, option C is the correct answer.