Diseases such as diphtheria result from a process called lysogenic conversion in which viral dna is integrated into a bacterial chromosome, which allows particular toxins to be expressed. what mechanism of gene transfer is involved?

The correct answer is horizontal gene transfer, transduction.  

Transduction (a type of horizontal gene transfer)is the process by which foreign DNA is introduced into a bacteria cell. When viruses infect bacterial cells, their DNA is integrated into a bacterial chromosome and thus harness the replicational, transcriptional, and translation machinery of the bacteria. As a result, bacteria cell makes numerous virions (viral particles).