Which class of protozoa is totally parasitic?

The correct option is D Sporozoa
Sporozoa:
The fifth phylum of the Protist kingdom, known as Apicomplexa, includes several species of obligate intracellular protozoan parasites classified as Sporozoa because they form reproductive cells known as spores. Many sporozoans are parasitic and pathogenic species, such as Plasmodium (P. falciparum, P. malariae, P. vivax), Toxoplasma gondii, Pneumocysts carinii, Coccidian, Babesia, Cryptosporidum (C. parvum, C. muris). The Sporozoa reproduction cycle has both asexual and sexual phases. The asexual phase is termed as schizogony (from the Greek, meaning generation through division), in which merozoites (daughter cells) are produced through multiple nuclear fissions.

Hence the correct option is B.