Frank avoids talking with food in his mouth because he knows others think it is socially inappropriate. This best illustrates the impact of

Social norms

Explanation:

Social norms are a collective understanding of what is acceptable or standard of behaviour accepted by the society. In other words, social norms are conducts which are generally considered normal by a group such as a cultural group or behaviours that are influenced by groups as big as a society. This is demonstrated above where Sam tries to stick to the acceptable norm of not Talking while eating.