Final answer:
A database is not always stored sequentially on disk primarily to improve data access and retrieval times by enabling the use of indexing and other efficient data retrieval techniques.
Step-by-step explanation:
The question asks why a database is not always stored sequentially on disk. The main reason is a) Improved data access and retrieval times. While storing data sequentially may seem efficient because it simplifies the design of the database management system, it can be quite inefficient in practice. Non-sequential storage allows for the use of indexing and other data retrieval techniques that reduce the need for long sequential reads and writes, thereby improving performance, especially in databases with large amounts of data or high transaction volumes. Furthermore, non-sequential storage can provide flexibility for additional aspects like growth and data reorganization without significant overhead.