The correct answer is 'a.' Key Points - The error lies in Part (a) of the sentence. - The verb 'think' is a stative verb. - Stative verbs describe a state rather than an action. - They aren't usually used in the present continuous form. - Example: I don't know the answer. - Therefore, the use of 'thinking' in Part (a) of the sentence should be replaced by 'thinks' to make it grammatically correct. - Correct sentence: My mother thinks that I should stop drinking.