The sentence is a conditional sentence, specifically a type of conditional known as the third conditional, which is used to talk about a situation in the past that didn't happen and its hypothetical result. The structure of the third conditional typically follows the pattern: "If + past perfect (had + past participle), ... would have + past participle." In this case, the sentence expresses a hypothetical situation in the past: "If I had seen him in the crowd." The correct option "had I seen" is the past perfect form of the verb "see" (had seen), which is used to indicate an unrealized or hypothetical past action. It is another version of the third conditional and it is used to make an utterance sound more rhetorical.