For many firms, the cheapest and most important source of equity capital is in the form of:. A - Debt
B - Preferred Stock
C - Retained Earnings

C - Retained Earnings

Explanation:

Retained earnings are  the cheapest source of equity capital for the following under listed points:

No costs of issuance of a new security in terms of underwriters' fees are required.

It is readily available, it would take days for the company to have access to the right amount of funding

The risk of under subscription where shares rolled for subscription are not subscribed in full is nip in the bud

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