Answer:
The best answer to the question: No one could believe their luck when the owner of the restaurant offered free dessert to all, What is the agreement error in this sentence? Would actually be, A: The pronoun "their" does not match its antecedent in number.
Explanation:
The reason for this being the correct answer is that no one indicates individuality and few in the number of subjects in the sentence. However, the pronoun that replaces this is a plural noun, indicating a group, or a big number. The combination of no one, and "their" as the pronoun that would replace that noun, is not coherent and therefore expresses a disagreement of number.
