Step 1: Understanding the question.
The question asks why 'X' is a leading brand in the 'Y' product category. To answer this, we need to understand both the customer behavior and the brand's marketing efforts.
Step 2: Analyzing the statements.
(I) Most of the customers of Y product category purchase 'X': This statement explains that customer behavior supports 'X' being a leading brand in the category, providing essential information for answering the question.
(II) 'X' has the highest advertising spend: This statement indicates that 'X' may be a leading brand because of its significant investment in advertising, which is another important factor in its success.
Step 3: Conclusion.
Both statements provide crucial information, making the correct answer (C) If both I and II are required to answer the question.