Step 1: Understand the context.
The sentence says his attitude made it difficult for friends to trust him. The blank requires a word that explains why someone is untrustworthy.
Step 2: Evaluate the options.
"Adjustable" (adaptable), "acceptable" (satisfactory), and "amicable" (friendly) are all positive traits that would not cause distrust.
Step 3: Select the correct word.
"Erratic" means unpredictable and inconsistent behavior, which logically makes a person difficult to trust. \[ \boxed{\text{erratic}} \]