Which of the following is an example of a no-additional-cost service exclusion?

The correct answer is the facilitation by a stock brokerage firm of the purchase of stock by employees. A no-additional-cost service is defined as a benefit provided by an employer that does not impose any significant additional costs on the employer, typically because it relates to services already offered to the general public.

In this particular example, the stock brokerage firm is facilitating a service that it already provides to clients and doesn't require any extra resources or expenses beyond what they would typically incur for their other customers. Because of this, the brokerage firm's action doesn't create additional costs for the business, making it fit the criteria of a no-additional-cost service exclusion.

In contrast, free train tickets, hotel rooms, or telephone services might involve significant costs for the provider to offer as free services. Each of these options typically entails direct expenses that the employer must absorb, which disqualifies them from being considered no-additional-cost services.