Topic: Group verb,

Best Answer Explained in Below-:

The phrase "Call in a doctor" is an idiomatic expression used in English to indicate that someone should summon or request the presence of a doctor, often for medical assistance. The preposition "in" implies that the doctor is being brought into a situation or location, which aligns with the context of needing professional help.

While the other options—"for," "at," and "by"—may seem plausible at first glance, they convey different meanings that do not fit as well in this context. For instance, "Call for a doctor" suggests that you are requesting the doctor to come or performing the action of calling, while "Call at a doctor" is not a standard expression. The phrase "Call by a doctor" could imply that a doctor should come to visit, but it feels less direct and is not commonly used in everyday language.

In contrast, "Call in" traditionally conveys an urgency or necessity to contact a professional, particularly in situations that require immediate medical attention. Therefore, "Call in a doctor" accurately captures the intent of summoning a doctor, making "in" the correct choice for completing the sentence.