These single-celled organisms can be pathogenic but some can also be edible.
a. bacteria
b. fungi
c. protozoa
d. viruses

The answer is a. bacteria.

Bacteria are single-celled organisms. Some species are pathogenic and are able to cause diseases such as tetanus, cholera, tuberculosis, etc. But, some of them can be "edible". This term should be used with caution. They are edible in terms that they are used in technology and industry. Some bacteria species are used in the production of cheese, yogurt, wine, etc.

The other three groups - fungi, protozoa, and viruses are also pathogenic. But, protozoa are not edible. Viruses are not edible and also not cellular. Fungi are edible, but the edible species are multicellular, not single-cellular.