The correct option is B (a)
Let’s examine the sentence: “He couldn't but help shed tears at the plight of the villagers rendered homeless by a devastating cyclone.” The helping verb ‘could’ needs an action verb to form any meaning. So, the verb ‘help’ should be moved before the conjunction ‘but.’ Also, two action verbs ‘help’ and ‘shed’ cannot go together as one ‘shed tears’ and not ‘help shed tears.’ So, the correct sentence would be, “He couldn't help but shed tears at the plight of the villagers rendered homeless by a devastating cyclone.” So, the error is in Part a of the sentence. Hence, Option A is the correct answer.