The correct answer is C. Internal business processes.
The fill rate is a measure of how well a company is able to meet customer demand for its products. Improving fill rate is directly related to the efficiency and effectiveness of a company's internal business processes, such as inventory management, production scheduling, and order fulfillment. Therefore, improving fill rate is an objective that falls under the Internal Business Processes perspective of the Balanced Scorecard.
The Financial perspective would focus on objectives related to profitability, revenue growth, and cost reduction. The Learning and Growth perspective would focus on objectives related to employee training and development, while the Innovation perspective would focus on objectives related to research and development, new product development, and market innovation.