The diphtheria exotoxin is an a-b toxin that view available hint(s) the diphtheria exotoxin is an a-b toxin that causes host cell lysis is encoded for by genes in the chromosome of corynebacterium diphtheriae cells is found in all corynebacterium diphtheriae cells is found only in corynebacterium diphtheriae cells that have been infected with lysogenic bacteriophage β

The correct answer is "is found only in Corynebacterium diphteriae cells that have been infected with lysogenic bacteriophage β".

An exotoxin is a bacterial toxin which is secreted by the toxin in its surrounding environment.
The diphtheria exotoxin is a toxin produced and secreted by some pathogenic bacteria of the species Corynebacterium diphtheriae. This exotoxin is only produced by bacteria who have been infected with lysogenic bacteriophage β (a specific type of virus which infects bacteria). The reason for this is that the virus encodes the gene responsible for producing the toxin.