doubled their value


I read the following question in a TOEIC book:

“104. Mr. Fullham’s investments in small start-up companies have doubled ______ value in just over five years.

(A) they

(B) their

(C) them

(D) themselves

(Answer: B)” ((p. 80, ETS TOEIC TEST vol.4))

In the above sentence, what does “their” refer to? “Mr. Fullham’s investments”? Or, “small start-up companies”?

Thanks in advance.
Niuewhat does “their” refer to?
Mr. Fullham’s investments. The subject of the sentence.

Thanks, CJ!