two actions are there on these questions.
first, the initial belief that wealth was less
and second that the belief was wrong
hence we have to use the word 'had' to denote the action which is more in past.
hence answer is E.

The recent discovery of Tutankhamen’s tomb by Egyptologists has provided information that suggests that the wealth accumulated by ancient Egyptian pharaohs was greater than believed.

2 actions - in the sub ordinate clause followed by that believe and the belief that is no longer believed. Hence what is no longer believed takes the had.

(A) believed.
missing auxiliary verb
(B) is believed.
new believe surpasses the old belief hence present tense is incorrect. we are talking about the old belief
(C) was believed before.
before is unnecessary
(D) they have believed before.
before is unnecessary
(E) had been believed.
Correct