In paragraph 11, the author says that Chang-bo had been fighting off doubts about the North Korean government for many years but that recently “those doubts were gelling into outright disbelief.

Adjective clauses  begin with words such as that, when, where, who, whom, whose, which, and why. an essential (or restrictive)  adjective clauseprovides information that is necessary for identifying the word it modifies. so for example*** wrong= i the man gave me direction. example right ( i the man who gave me the  direction.

Bwould be the answer