In this idiom question, choose the option that best expresses the meaning of the phrase: Call in question.

Introduction / Context:
The phrase call in question is a common idiomatic expression in formal English, especially in legal, academic, and journalistic writing. It is often used when the validity, truth, or reliability of something is being examined critically. Knowing this idiom helps you understand serious discussions and written analyses.

Given Data / Assumptions:
- Idiom: Call in question. - Options: Summon as a witness, Doubtful, Prove a theory, Challenge. - No additional context is provided, so the meaning must be known from prior learning.

Concept / Approach:
To call something in question means to challenge it, to express doubt about it, or to dispute its correctness. It does not mean to summon someone physically, nor does it mean to prove something. The central idea is raising objections or doubts about the matter under discussion. Therefore, challenge is the closest and most accurate one word equivalent among the options.

Step-by-Step Solution:
Step 1: Identify that call in question is used when we doubt or dispute something. Step 2: Compare this idea with the options given. Step 3: Summon as a witness refers to calling a person to appear in court, not questioning the truth of a statement or idea. Step 4: Doubtful is an adjective describing a state of being uncertain; it is not a verb expressing the action. Step 5: Prove a theory means to confirm or establish its truth, which is the opposite of questioning it. Step 6: Challenge as a verb means to dispute, question, or contest something, which aligns directly with call in question.

Verification / Alternative check:
Consider sentences such as The new evidence calls his honesty in question or The report calls into question the safety of the product. If we replace calls into question with challenges, the meaning remains the same: The new evidence challenges his honesty. This confirms that challenge correctly captures the essence of the idiom. None of the other options can replace the phrase without changing the sentence fundamentally.

Why Other Options Are Wrong:
Summon as a witness describes a legal act of asking someone to appear in court, not of doubting a claim. Doubtful is not an action but a descriptive word, and it does not stand as a substitute for call in question in sentences. Prove a theory is the opposite of questioning it; it means to establish the theory as true, not to call it into doubt.

Common Pitfalls:
Students sometimes focus on the words call and question separately and assume it must involve calling a person to be questioned, leading them to choose summon as a witness. The full idiom, however, focuses on the object whose truth is being challenged, not on summoning people. Always think of the whole phrase call in question as equivalent to challenge or cast doubt upon rather than analysing each word literally.

Final Answer:
The idiom Call in question means to Challenge.