To answer this sort of multiple choice question about T. S. Eliot's play Murder in the Cathedral, you should think through each of the possible choices carefully, and begin by eliminating the obviously impossible.
The second answer, "b/ historical drama", isn't actually a genre. Plays in many different literary forms can be written about periods in the past of the playwright. Shakespeare's history plays such as Henry IV or Richard III, Aeschylus' Persians, Arthur Miller's The Crucible, and A Funny Thing Happened on the Way to the Forum are all, in their ways, historical dramas, but are written in quite different genres using different styles of staging.
The use of the chorus and limited number of actors engaging in dialogue resemble to a degree the staging of Greek drama, as does the heroic but flawed protagonist. However, Murder in the Cathedral lacks the elements of singing and dancing typical of Greek drama.
Although the subject matter of Murder in the Cathedral is medieval, it lacks many of the generic elements of the morality play. It does concern ethics, but its protagonist is a fully individuated historical character. Some of the choices made by Beckett are moral, but there is far greater ambiguity in the moral choices and the characters than one would find in an actual Tudor drama and the versification more fluid and sophisticated. However, there are certain elements of the morality play present in Eliot's work.
Overall, (c) is probably the closest answer, but you could also argue for (d), in that Eliot, while borrowing certain elements from Tudor drama, does not strictly follow Tudor conventions of staging, characterization, or versification.