To determine the correct statements, let's analyze each given option by understanding the concepts of flow and stock, and depreciation.
(A) Depreciation is a flow concept: A flow concept measures quantities over a period of time, such as income or expenditure. Depreciation, calculated as the annual reduction in value of an asset due to wear and tear, fits this definition. Thus, this statement is correct.
(B) Money supply is a stock concept: Stock concepts measure quantities at a particular point in time. Money supply, the total amount of monetary assets available in an economy at a particular time, is indeed a stock concept. This statement is correct.
(C) Investment is a stock concept: Investment refers to the accumulation of physical assets or capital over time, making it a stock concept. However, the change in capital over a time period is a flow concept. Since investment here refers to the amount at a particular point, it can be considered a stock concept. Thus, this statement is correct.
(D) Depreciation is an annual allowance for wear and tear of a consumer good: Depreciation refers to the reduction in value due to wear and tear, age, or obsolescence. This often relates to capital goods, rather than consumer goods, but generalizing as "an annual allowance" fits the broader idea of depreciation. This statement is correct.
All the statements (A), (B), (C), and (D) are correct.