Read each of the following sentences to find out whether there is/are an error…

Concept

In a copular question, the finite form of “be” agrees with the grammatical subject and moves before that subject through subject–copula inversion. Within a noun phrase, an attributive adjective describes the head noun, while a related abstract noun names the quality or state itself.

Application

1. In “What is this mad behavior?”, the noun phrase “this mad behavior” is the singular subject, “is” is the agreeing copula placed before it, and “what” functions as the subject complement.

2. “this” is the singular demonstrative determiner that introduces the subject noun phrase.

3. The source segment “madness behavior?” uses the abstract noun “madness” where the quality must be expressed by the adjective “mad”. Replacing it produces “What is this mad behavior?”

4. The singular head noun “behavior” and the final question mark then complete the interrogative sentence.

Contrast

• “What is” supplies the inverted copula and interrogative complement.

• “this” supplies the singular demonstrative determiner.

• “madness behavior?” contains the noun-for-adjective substitution that requires correction.

• “More than one of the above” would require at least two erroneous segments, but the analysis isolates one.

• “None of the above” would require every segment to be acceptable, but the noun-for-adjective substitution remains.

Result

The erroneous underlined part is “madness behavior?”; replace “madness” with “mad” to obtain “mad behavior”.