This question evaluates the understanding of marketing concepts. 'Adding stickiness by amusing, entertaining content' is about making the product or content more engaging to encourage repeat engagement. 'Brand loyalty' (A) is the result of such strategies, but it's not the act itself. 'Customer retention' (B) is directly related to making content engaging to keep customers. 'Market saturation' (C) refers to the point at which the market cannot absorb more of a good or service. 'Product differentiation' (D) is about distinguishing a product from others, not necessarily about entertainment.