Which of the following describes an open shop?

An open shop refers to a workplace where employees can be hired without regard to their union membership status. This means that both union and non-union workers are allowed to work alongside each other, which provides flexibility for employers and employees. In an open shop, individuals are not compelled to join a union as a condition of employment, thereby allowing a more inclusive hiring practice. This setup can promote a diverse workforce and may appeal to employees who prefer not to join a union due to personal beliefs or financial considerations.

In contrast, the other options focus on scenarios where union membership is either mandatory or exclusive to employment conditions, which directly contradict the definition of an open shop. Therefore, the correct understanding of an open shop centers around the freedom of choice regarding union affiliation for workers.