Final answer:
Primates communicate affection and reduce group tension through touch and sight.
Step-by-step explanation:
The common method by which primates communicate affection and reduce group tension is through touch. Many primates, including Indian langurs and great apes, engage in activities such as grooming, touching each other's shoulder or tail, embracing, lip contact, and greeting ceremonies to express affection and alleviate tension within the group. Additionally, primates also use sight as a means of communication, using gestures and facial expressions to convey messages to other members of their species.