a) Data expiration is useful for some classes of information, including machine generated event data, logs, and session information
b) A special TTL index property supports the implementation of TTL collections
c) TTL collections make it possible to store data in MongoDB and have the mongod automatically remove data after a specified number of seconds or at a specific clock time
d) All of the mentioned

Answer: d
Explanation: The TTL feature relies on a background thread in mongod that reads the date-typed values in the index and removes expired documents from the collection.