Concept:The sentence describes trust shared between people in a relationship.
Explanation:Trust in human relationships involves two or more people.
The blank needs a word that means a feeling or action done by each person toward the other.
"Mutual" means shared equally between two or more parties.
"Communal" refers to a whole community, not a direct two-way relationship.
"Individual" and "personal" both suggest a single person, which does not fit the context.
Thus, the most suitable word is "mutual".
Answer:C. mutual trust.