Answer:
C. hidden curriculum.
Step-by-step explanation:
Hidden curriculum refers to the value or knowledge that being taught by the schools through indirect or informal methods. The knowledge from hidden curriculum tend to be taught through things such as subtle language, treatments toward a certain group or political views, extracurricular activities, etc.
The teacher's action is considered a hidden curriculum because when a teacher decorates the classroom only for a certain religion while not giving the same treatment to the others, that teacher basically taught the students to value that certain religion over the other without saying it directly.