Final answer:
The event when one piece of data has a hash value of another piece of data already in the hash table is called a hash collision.
Step-by-step explanation:
The correct answer is a) Hash Collision. When two different pieces of data produce the same hash value and are stored in the same slot of a hash table, it is called a hash collision. This can happen because the number of possible hash values is limited, while the amount of data that needs to be stored can be very large.
For example, let's say we have a hash table with 10 slots, and we want to store two pieces of data: 'apple' and 'banana'. Both of these words produce the same hash value of 3. Since both words have the same hash value, they will be stored in the same slot of the hash table, causing a hash collision.