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.
References:
Working with Very Large SOQL Queries
Apex Governor Limits