This is quite an interesting question- the key identifier is the pronoun they. They refers to the guests and not the goons breaking in so
(A) had had plenty of time to glut themselves at the feast by the time the rebels broke into the castle, and thus
they, This they can refer to any one of the rebels or the guests. Hence wrong choice.
(B) had plenty of time to glut themselves at the feast by the time the rebels broke into the castle, and thus
they: Same problem as A
(C)had plenty of time to glut themselves at the feast by the time the rebels
had broken into the castle, and thus the guests
Had broken, no the actions occurred at that instant, not in the past. Thus it should be simple past broke into.
D)had had plenty of time to glut themselves at the feast by the time the rebels broke into the castle, and thus the guests : YAY, Mr right answer

(E)had plenty of time to glut themselves at the feast by the time the rebels were breaking into the castle, and
furthermore they: Both furthermore and they, what the hell is happening

Hope this helps !