Final answer:
With a reserve ratio of 20%, the bank must keep $2,000 in reserves from the $10,000 demand deposit it received.
Step-by-step explanation:
The reserve ratio is the fraction of deposits that a bank is required by the Federal Reserve to hold as reserves, rather than loaning out or investing. If a bank receives $10,000 in demand deposits and the reserve ratio is 20%, then the bank must hold 20% of $10,000, which is $2,000, in reserves. Therefore, the correct answer is D. $2,000.