The part of the sentence that tells "who" or "what" the sentence is about is the A. direct object
B. indirect object
C. predicate
D. subject

D, Subject

Explanation:

Took a gamble with the prior answer and D, got it correct though.

(For future people that I know will be arriving shortly here.)

Screenshot

1. it was caused by feudalism

2. charlemagne used christianity as a major cornerstone for his rule. cloisters were built all over the empire to act as centers for the (re)population of the vast european wilderness.

3. the europeans feared the next raid every time chetak

i'm not so sure about #6. but i hope i you with the first 3.