Concept:The fixed phrase "cannot/could not help but + base verb" means to be unable to avoid doing something.Explanation:The error is in part (a).The correct idiomatic expression is "couldn't help but" or "couldn't help + verb‑ing"."Couldn't but help" is wrong because "help" should come after "but" or be followed by the gerund.In this sentence, the correct form is "couldn't help but shed".Thus, part (a) has an error.Answer:A (a)