Which code snippet processes records in the most memory efficient manner, avoiding governor limits such...

Option D, which uses the Database.query() method in combination with a for loop, is the most memory-efficient way to process records. This is known as the Query Locator pattern, which allows you to work with a large number of records by querying them in batches. This approach helps to avoid hitting the governor limit for "Apex heap size too large" as it processes one record at a time from the database without storing the entire result set in memory.