Spot the part that has error in it. Each of the speakers whom I invited (A) /…

Concept:

Under traditional, formal prescriptive grammar — the register 'spot the error' items in competitive exams test — indefinite pronouns like each, everyone, everybody, anyone, and nobody are treated as singular. Any pronoun used later in the sentence to refer back to one of them must therefore also be singular (his, her, or the gender-neutral his or her), not a plural form such as their, they, or them, even when the sentence is talking about several people.

Application:

Here the subject is 'Each', which is singular. The segment marked C — 'indicated their unwillingness' — uses the plural possessive 'their' to refer back to this singular subject, so it breaks pronoun-antecedent agreement under this rule. Replacing 'their' with 'his' (or the gender-neutral 'his or her') — 'indicated his unwillingness' — removes the error.

Checking the other parts:

• A — 'whom I invited': 'whom' is the object form of 'who' and correctly serves as the object of 'invited' ('I invited the speakers'), so there is no case or agreement error here.

• B — 'to participate in the debate has': the singular verb 'has' correctly agrees with the singular subject 'Each', and the infinitive phrase is grammatically sound.

• D — 'because of the short notice': a straightforward prepositional phrase with no subject-verb or pronoun agreement involved.

Since only the segment marked C has a pronoun that disagrees in number with its antecedent under this rule, that is the part containing the error.