Final answer:
Transaction processing systems in large enterprises typically use relational databases to store their data because they support a large volume of structured data and complex transactions.
Step-by-step explanation:
The question pertains to the type of database systems used to store data in large enterprise transaction processing systems. Typically, these systems require databases that can handle a large number of transactions and provide high levels of reliability, consistency, and availability.
While NoSQL databases are growing in popularity for certain use cases, large enterprises often rely on relational databases for their transaction processing systems. These databases are designed to handle a large volume of structured data and support complex queries and transactions.
Therefore, the answer to the question is: d. Relational database.