Answer:
A. isolation.
Step-by-step explanation:
Isolation is not a sign of cognitive mental stress, because it doesn't imply a cognitive result.
In psychology, isolation is a defence mechanism, because it consist in minimizing any associative connection with others. In addition, when someone is stressed cognitively, it's said that people is constantly worrying, has racing thoughts, poor judgement, the person is not capable to focus, sees only negative sides of situations, even the person experience disorganization and forgetfulness, because the stressful state doesn't allow to have normal cognitive processes. However, isolation is not a result for this stress, because all negative impacts are around mental processes.
Therefore, the correct answer is A.