Concept:After verbs like "promise", the infinitive must use the base form of the verb.Explanation:In the phrase "to delivered", the word "delivered" is the past tense or past participle, not the base form.The correct infinitive should be "to deliver".Therefore, the error is in part (a).Parts (b) and (c) are grammatically correct.The corrected sentence would be "They had promised to deliver the bags before mid-March to enable us to inaugurate the shop."Answer:A