Introduction:
ACID stands for Atomicity, Consistency, Isolation, and Durability. These are the key properties that ensure the reliability and integrity of database transactions. Each of these properties plays a crucial role in maintaining the consistency and reliability of the data.

Explanation:
a) Atomicity:
Atomicity ensures that a transaction is treated as a single, indivisible unit of work. It means that either all the operations within a transaction are executed successfully, or none of them are executed at all. If any operation within a transaction fails, the entire transaction is rolled back, and the database is restored to its original state.

b) Consistency:
Consistency ensures that a transaction brings the database from one consistent state to another. It means that the data remains consistent before and after the transaction. The integrity constraints, such as unique key constraints and referential integrity, are maintained during the execution of the transaction.

c) Isolation:
Isolation ensures that concurrent transactions do not interfere with each other. Each transaction is executed in isolation, as if it is the only transaction running in the system. This prevents data inconsistencies and ensures that the execution of one transaction does not affect the execution of other transactions.

d) Deadlock-freedom:
Deadlock refers to a situation where two or more transactions are waiting for each other to release resources, resulting in a circular dependency. Deadlock-freedom is not a part of the ACID properties. Deadlock prevention and detection mechanisms are typically implemented separately in database management systems to avoid deadlocks.

Conclusion:
In summary, the ACID properties of database transactions are Atomicity, Consistency, Isolation, and Durability. Deadlock-freedom is not a part of the ACID properties. The ACID properties collectively ensure that database transactions are reliable, consistent, and maintain data integrity.