Final answer:
The correct answer is B. grown, which is the past participle needed to complete the sentence in the past perfect tense.
Step-by-step explanation:
To complete the sentence using the past participle of the irregular verb 'grow,' the correct form is 'grown.' Therefore, the accurate sentence is 'After the little girl had grown up, she began to realize all that her parents had done for her when she was young.'
Past participles are used with perfect tenses, and 'grown' is the past participle form of the verb 'grow.' It is important to distinguish between the simple past tense 'grew' and the past participle 'grown' when forming perfect tenses, such as the past perfect tense used in this sentence.