What form of industrial action involves a group of workers withdrawing their labor for a specific purpose?

The form of industrial action that involves a group of workers withdrawing their labor for a specific purpose is a strike. During a strike, employees collectively choose to stop working as a way to protest against their employer, often to demand better pay, improved working conditions, or to express dissatisfaction with management practices. Strikes are typically organized and can involve significant planning, as they require solidarity among workers to be effective.

In contrast, other forms of industrial action, such as a protest or walkout, may not officially involve a sustained withdrawal of labor. A protest usually refers to a demonstration or gathering aimed at raising awareness of certain issues, but it doesn't necessarily require the workers to stop performing their jobs. A walkout often indicates an abrupt, usually unexpected action where workers leave their posts, but it might not always carry the same formal intent or organization behind it as a strike. Work to rule involves following all regulations and procedures strictly, which can slow down productivity but doesn't represent a complete withdrawal of labor. Thus, the definition of a strike aligns precisely with the withdrawal of labor for a focused purpose, making it the correct answer.