Ghost Writing Occurs Under Which of These Circumstances

Ghost writing occurs under which of these circumstances?

A) a professor who did not work on the research is made an author of the published results
B) too many researchers work on a study and so some are left off as authors on the published results
C) professors or researchers have their work written by someone else, but the professor's name appears on the work
D) a professor publishes the unpublished results of another professor's study and claims it as their own