Final answer:
The internal source of liquidity from the given options is d. Sales of products or services, as it is generated from within the firm's own operations.
Step-by-step explanation:
The internal source of liquidity among the options provided is: d. Sales of products or services. Internal sources of liquidity are those that are generated within the firm's own operational activities.
Borrowing, such as through bank loans or bonds, and sales of stock are external sources because they involve receiving capital from outside entities. Gifts and donations can be considered external as well since they come from third parties and not from the firm's business operations.