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Answer:
Vicarious reinforcement
Explanation:
The increase in Danny's violent behaviour is attributed by the behavioural theorists to a vicarious reinforcement because Danny recognises the fact that a rival gang member was senselessly murdered by their gang leader and still got rewarded and respected by the gang members thereby imitating the actions of his gang leader because he saw that the gang leader was greatly rewarded after killing a rival gang member
I believe the answer is: Primary
For example, let's say take a look at normal employees at a company.
For most of them, the main reason they spend their time to do the job correctly is to obtain money/salary even though they have to spend their time being scolded by unlikable boss.
In the scenario above, we can say that money is the primary reinforcer for the target behavior