The thing about which Nick was surprised when he first met Gatsby at the party in chapter 3 is that Gatsby is a young man.
The themes of The Great Gatsby idealism, class, wealth and love.
When Nick saw Gatsby for the first time in chapter 3, he made a remark “I was looking at an elegant young rough-neck, a year or two over thirty," which clearly states that Gatsby is young.
Hence, the correct option is A that is "that Gatsby is a young man".
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