The most appropriate one-word substitution for the given group of words "An act of ending the employment of a worker or group of workers" is "Layoff."

Explanation of the options:

☀ Expelled: This term typically refers to being removed or forced out of a place or organization, but it doesn't specifically capture the act of ending employment.

☀ Discontinued: This term generally means to cease or stop something, but it doesn't specifically pertain to ending employment.

☀ Layoff: This term accurately represents the act of terminating the employment of a worker or a group of workers due to various reasons, such as economic downturn, restructuring, or downsizing.

☀ Terminating: While this term can be related to ending employment, "layoff" is a more specific and commonly used term in this context, denoting the act of dismissing employees.