The correct answer is D. Retained earnings.
Retained earnings are the accumulated net income of a company that has not been distributed to shareholders as dividends. They are a source of internal financing that can be used to fund working capital needs, such as the purchase of inventory or the payment of accounts payable.
Public deposits, shares, and debentures are all sources of external financing. Public deposits are funds that are raised by a company through the sale of shares to the public. Shares are units of ownership in a company. Debentures are loans that a company takes out from a lender.
While all of these sources of financing can be used to fund working capital needs, retained earnings are the most stable and reliable source. This is because they are not subject to the same fluctuations as other sources of financing, such as public deposits or shares. Additionally, retained earnings do not require a company to pay interest or dividends, which can save the company money.
Therefore, the main source of working capital is retained earnings.