Diphtheria is caused by

The correct option is A Poisons released by living bacterial cells into the host tissue
Diphtheria is a serious bacterial infection usually affecting the mucous membranes of the nose and throat. The bacterium Corynebacterium diphtheriae causes diphtheria by releasing a poison that can cause grayish membrane to form and coat the inside of the victim's throat. The membrane can cause breathing problems.
Therefore, the correct answer is option A.