Final answer:
The underlined word is defined as a preconceived opinion not based on actual fact.
Step-by-step explanation:
The underlined word in the sentence 'Shelley was convinced that Allan felt strongly towards her because of a long-held prejudice, not something she had actually done wrong' is defined as a preconceived opinion not based on actual fact. In this context, Shelley believed that Allan's feelings towards her were based on a prejudice he held, rather than any actual wrongdoing on her part. This definition aligns with option D, 'a preconceived opinion not based on actual fact'.