Concept:The error is a grammatical mistake involving a possessive adjective.Explanation:The phrase "expressing you regret" is incorrect.The word "you" is a subject or object pronoun.Before the noun "regret", we need the possessive adjective "your".Thus, the correct phrase is "expressing your regret".The corrected sentence reads: "Write an email to your friend expressing your regret for not being able to attend his birthday party."Parts (a) and (c) have no errors.Answer:(b)