What sentiment is expressed by the man in response to the woman's comment about being an expert?

The sentiment expressed by the man in response to the woman's comment about being an expert is one of wistfulness or longing, which aligns with the choice of "If only." This phrase typically conveys a feeling of regret or desire that something were different, often reflecting on a situation that one wishes could be changed or improved. In this context, the man's response may imply that he admires her expertise but perhaps wishes he could attain the same level of skill or knowledge, or he might be expressing a sense of wishing for a scenario where that expertise could be beneficial in some way. This emotional nuance captures the notion of wanting something beyond reach, making "If only" the most fitting choice for expressing a contemplative sentiment.