Answer:
A firm that requires union membership by its workers as a condition of employment.
Step-by-step 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.