The primary purpose of banning ultra-thin models is to protect models from health risks associated with extreme dieting. This ban aims to send a message to the fashion industry about the responsibility it has in shaping societal standards of beauty, particularly for young women. Option A is incorrect because the ban is not about promoting a specific body type. Option C is misleading as the focus is on health, not sales. Option D does not address the health implications that the ban seeks to mitigate.