Which of the following statements is true regarding transactions in a database?

The statement that a transaction logs changes made to the database is true because transactions are fundamental operations in database management systems that ensure data integrity and consistency. When a transaction is executed, it typically involves a sequence of operations that modify the state of the database. To maintain a record of these changes and to enable recovery in case of failure, many database systems implement transaction logging. This logging allows the system to track which changes were made during the transaction and assists in maintaining the "ACID" properties (Atomicity, Consistency, Isolation, Durability) that are crucial for database transactions.

The other options do not accurately reflect the characteristics of transactions. For instance, not all transactions require foreign keys; they can involve various types of data manipulations regardless of relationships defined by keys. Additionally, transactions can occur on disk as well as in memory, as database operations often involve persistent storage. Lastly, transactions are closely related to the structure of the database since they operate on tables, relationships, and data schemas, making the notion that transactions are unrelated to database structure incorrect.