The best answer is B.
Bacteria can be agents of disease. Diseases caused by bacteria are called bacterial infections. Examples are typhoid which is caused by a bacteria called Salmonella and sore throat which is cause by a bacteria called Streptococcus
Viruses are smaller than bacteria but they have to invade and enter cells in order to live. They can cause diseases such as HIV Aids and Hepatitis.
Protozoa are unicellular organisms mostly found in water. An example is amoeba which can cause amoebic dysentery.
Fungi are organisms which feed on dead organic matter. They can cause diseases such as Athletes foot in man.
