Which strains of C. diphteria are causing Diphteria toxin?
Real simple: Beta!  Often called "phage beta" b-prophage is a more modern way to write it.  I knew that one off the top of my head, but I like to refer the general public interested in bacteriology to Tadar's online text.

It's nicely done, and I wish that it had been around when I was studying bacteriology.

Specific to your question:


  Hence, Corynebacterium diphtheriae can only produce the toxin
  responsible for the disease if it carries a temperate virus called
  phage beta. Only lysogenized streptococci  produce the erythrogenic
  toxin (pyrogenic exotoxin) which causes the skin rash of scarlet
  fever; and some  botulinum toxins are synthesized only by lysogenized
  strains of C. botulinum.