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Explanation:
The hairdressers have agreed on the prices. So, changing the price in any manner would be violation of the agreement. Thus, options A,
B, C incorrect. Note that option C is making the price 0 which is also in violation of agreement.
In decision making questions, it is important to focus on the premise of the question. The central purpose of the shopkeeper is to avoid any violation of agreement while retaining his loyal customers. The option that fits both is the best fit.
Hence option D is the best fit among the given options where loyal customers are benefited while not violating the agreement.
It is entirely true that jumping queue is a moral quandary in itself but as per the central purpose of the author, option D is the lesser of the evils and logically coherent.
Hence, option D is the correct answer.