Y is it right ?

Shady Ghany, let's examine the other choices:

(B) "Each of the book's protagonists" is the subject. Each is a singular indicator, so the verb has to be singular. So B is not the right choice.
(C) "All ...". All is a plural indicator, so the verb has to be in plural format, i.e., "have". C is not the right choice.
(D) This is not a good structure because the sentence has two subjects: "Mark ..., Mary,..." and "each", so it is not a good choice.
(E) "could have had" is not the appropriate tense for a general truth like having a dynamic personality. General truths are typically represented in present tense.

Hence, the original sentence is alright. (A).

Why is it has and not have referring to book's protagonists?

Thanks

Em, it is "has", because the subject is "each of ...". Each is singular.

A