Final answer:
Reflexive behavior is the correct term describing collective actions executed quickly without a deliberate decision-making process, exemplified by automatic responses such as the knee-j erk reflex.
Step-by-step explanation:
Collective actions executed quickly without applying a deliberate decision-making process describes Reflexive behavior. Reflexive actions are involuntary responses to stimuli that occur naturally and are not influenced by conscious thought. For example, the knee-j erk reflex is a classic illustration where the leg involuntarily extends when the patellar tendon is struck.
However, collective actions such as flash mobs are examples of collective behavior, which comprises various non-institutionalized group activities that people engage in voluntarily, such as crowds, masses, and publics. Nevertheless, these do not describe the impromptu, involuntary actions characterized by reflexive behavior.