In the following question, a statement has been given with highlighted text.…

Concept: An idiom is a fixed group of words whose meaning is figurative and settled by usage; it cannot be worked out from the literal meanings of the individual words. In a replacement task the idiom required is the one whose established meaning is equivalent to the phrase being replaced and which fits the same grammatical slot — here, a noun phrase describing a person.

Application: The phrase to be replaced, “a weak person”, is a noun phrase for someone who lacks strength, firmness or substance. The idiom “man of straw” carries exactly this settled meaning — a person of no real strength, substance or authority. The image behind it is a straw figure that has a man’s shape but no force of its own. Substituting gives: “The king, being a man of straw, never went to the battlefield.” — which keeps both the sense and the grammar of the original sentence.

Cross-check: how the other expressions differ.

• “Man of letters” means a scholar or writer; it speaks of learning, not of any lack of strength.

• “Last straw” means the final small addition that makes an already heavy burden unbearable; it names an event, not a person.

• “Daredevil” means a recklessly bold person who courts danger; it speaks of excess daring.

Result: the completed sentence reads “The king, being a man of straw, never went to the battlefield.”