Which of the following is NOT a source of external capital?

Cash from operations is the correct choice as it is not considered a source of external capital. Instead, it represents internally generated funds from a company's ongoing business activities, primarily from the sale of goods and services. This capital is retained within the business and is used to cover expenses, reinvest in operations, or distribute as dividends, but it does not involve raising funds from outside sources.

On the other hand, bank debt, common stock, and preferred stock are all classified as external sources of capital because they involve obtaining funds from external investors or lenders. Bank debt is money borrowed from financial institutions, common stock represents ownership shares sold to investors, and preferred stock is a type of equity security that typically provides a fixed dividend and has priority over common stock in asset distribution. These forms of capital entail obligations or commitments to parties outside the company's operational framework, contrasting with cash derived from internal workings.