The correct answer is A. to improve disk performance.
A block cache or buffer cache is a data structure used in computer operating systems to improve the performance of disk-based I/O. The cache stores a copy of recently accessed disk blocks in main memory, so that they can be read or written more quickly than if they had to be read from or written to the disk.
This can be a significant improvement, as disk I/O is typically much slower than memory access. By caching frequently accessed data in memory, the operating system can avoid having to make as many disk requests, which can significantly improve the overall performance of the system.
Options B, C, and D are incorrect because they are not the primary purpose of a block cache. Interrupts are handled by the interrupt controller, and the capacity of main memory is increased by adding more memory modules.
Option E is also incorrect, as block caches are a common feature of most operating systems.