Answer:
Injunctive norms.
Step-by-step explanation:
An injunctive norm can be defined as those unwritten sets of beliefs and rules that are supposed to be approved or disapproved by others. It is a perception of approved and disapproved behaviours.
In the given case, the norm that one should not litter is injunctive. It is an injunctive norm because though it is not written in a law or rules but it is perceived to be an approved behaviour that one should not litter as it hampers the beauty of the environment and create an unclean environment.
So, the correct answer is an injunctive norm.