This talks about the survey and the company's success in past.
Rest mentions have/having/feeling... in present.
So i choose option D, by my reasoning.
Awaiting some more insights on this...

Regards
Sony
_________________

D for me..

Reason:
A) due to the fact that its customers felt limited in their purchasing options than having brand loyalty...wrong..1)due to the fact..less precise..2)parallelism...than is used to compare 2 things and both of these should be parallel..Wrong..

B) due to its customers feeling limited in their purchasing options than to having brand loyalty...The sentence incorrectly states that the company had enjoyed success due to its customers feeling...WRONG..changes meaning of the original sentence..

C) because its customers felt limited in their purchasing options than that they have brand loyalty..wrong..parallelism problem..parts compared..because its customers...than that..not parallel..also..what THAT is doing in second part of the comparison??..what does it refer to????...wrong..

D because its customers felt limited in their purchasing options than because they had brand loyalty..correct..no problems..sentence is parallel..BEST option..

E because of its customers feeling limited in their purchasing options than because of their having brand loyalty..Same as B..changes meaning..wrong..

The series of surveys revealed that the company had enjoyed success more due to the fact that its customers felt limited in their purchasing options than having brand loyalty

A due to the fact that its customers felt limited in their purchasing options than having[who have?] brand loyalty
Wrong.
- due to the fact is not correct.
- Structure is not parallel
- Who have brand loyalty?

B due to its customers feeling limited in their purchasing options than to having brand loyalty
Wrong.
- Not parallel. [due to X than to Y] is wrong idiom.

C because its customers felt limited in their purchasing options than that they have brand loyalty
Wrong.
- "That" refers to what?

D because its customers felt limited in their purchasing options than because they had brand loyalty
Correct.

E because of its customers feeling limited in their purchasing options than because of their having brand loyalty
Wrong. Awkward.

What is "they" referring to in option D the other choices didn't make sense and I picked D but I want to know what is the referent for They..

To answer your question, I searched the definition of "brand loyalty"

According to Wikipedia: "Brand loyalty is where a person buys products from the same manufacturer repeatedly rather than from other suppliers"

Who have brand loyalty? Only customers do. So "they" clearly refers to customers.

Hope it's clear.