Which of the following is a form of common external capital available?

Preferred stock is considered a form of common external capital available to a company because it is issued to outside investors and represents an ownership stake in the company, albeit with characteristics distinct from common stock. Preferred stockholders typically receive dividends before common stockholders and enjoy a higher claim on assets in the event of liquidation.

Retained earnings, on the other hand, refer to the accumulated profits that a company reinvests back into its operations rather than distributing them as dividends to shareholders. Because they are generated internally, retained earnings are not classified as external capital.

Asset depreciation relates to accounting practices that allocate the cost of tangible assets over their useful lives; it does not represent a form of capital but rather an expense and a decrease in asset value.

Uniform accounting practices are standards that ensure consistency in financial reporting across companies but do not constitute capital themselves. Thus, among the options presented, preferred stock is the clear choice as it directly aligns with the definition of external capital.