Final answer:
Unhealthy behavior is mostly learned through Observational learning, which involves observing and imitating others' behaviors, especially when they are rewarded.
Step-by-step explanation:
Unhealthy behavior is learned in a few different ways, but when it comes to how most behaviors are learned, the correct answer would be C) Observational learning. Observational learning occurs when individuals observe and replicate the behavior of others. This type of learning is particularly potent when the observed behavior seems to lead to a reward or desirable outcome. An example of this is seeing peers engage in unhealthy eating habits and subsequently adopting those habits because they appear to be associated with social acceptance or enjoyment. Both classical conditioning and operant conditioning are also forms of learning, but they're more about forming associations between stimuli and responses, or between behaviors and their consequences, respectively, rather than directly imitating another's behavior.