Meg needs an electrician to do some work at her home. Her father advises her to make sure that the firm she plans to hire is not a closed shop. Meg is not sure what this means, but later learns that the "closed shop" that her father was referring to is:.

A firm that requires union membership by its workers as a condition of employment.

Explanation:

As the exercise suggests, a "closed shop" is a firm which requires union membership by its workers as a condition of employment be it to be hired or to remain at said firm, as a form of union security. So, in the example, what Meg's father is asking is that she makes sure that the firm the hires is not a closed shop, therefore, their workers does not necesarilly have to be in the union to be able to work.