Final answer:
The firm's executives are not an external source for new-product ideas; rather, they are part of the internal structure. External sources typically include customers, suppliers, competitors, and industry events such as trade shows and magazines.
Step-by-step explanation:
The question is seeking to identify which option is not a common external source for new-product ideas in the context of a business. Among the options provided, the firm's executives are not considered an external source, as they are part of the internal structure of the firm. Common external sources for new-product ideas include customers (A), suppliers (B), competitors (C), and trade shows and magazines (D) since external sources refer to any entity outside of the firm's own organizational structure.