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Equestrian is the only Olympic sport in which men and women compete together.
Literal
Equestrian-competition [topic-は] men-women mixed [in-で] is-held only-の Olympic event is.
A sports-trivia sentence. 馬術 (ばじゅつ) is the formal term for 'equestrianism,' 'horsemanship' as a sport. 男女混合 ('mixed men-and-women') is a compound: 男女 (men and women) + 混合 (mixed, combined). The passive 行われる ('is conducted, is held') is the standard verb for events/competitions. The relative clause 男女混合で行われる ('that is held with men-and-women mixed') modifies 唯一のオリンピック競技 ('the only Olympic event') — 唯一 means 'only,' 'sole,' 'singular.' The whole sentence is a good example of how Japanese packs dense description into a single noun-modifier clause.