D is correct. Economic capital is a bank's own estimate of the capital it requires. In both cases, capital can be thought of as funds that are available to absorb unexpected losses. A common objective in calculating economic capital is to maintain a high credit rating. Economic capital is allocated to a bank's business units so that they can be compared using a return on allocated economic capital metric.

Option A is incorrect because the regulatory capital for credit risk is designed to be sufficient to cover a loss that is expected to be exceeded only once every thousand years, not every ten years as stated.

Option B is incorrect as equity capital is sometimes referred to as going concern capital because it absorbs losses while the bank is a going concern (i.e., it remains in business), not regulatory capital.

Option C is incorrect because the most important capital is equity capital, not regulatory capital. Regulatory capital is set by external regulatory bodies to ensure the stability of the financial system, while equity capital represents the ownership interest in the bank and is used to absorb losses and provide a buffer for unexpected events.