The young man asked his parents not to worry because he was full of optimism about his career.
【正确答案】 A
【答案解析】解析：句中词“optimism”表示“乐观，乐观主义”。A项表示“自信，信心”；B项表示“动机，动力”；C项表示“想象，想象力”；D项表示“决心，坚定”。A项的意思与原文相符，所以应该选A。