Frederich August Kekule discovered that the structure of benzene is a:

Friedrich August Kekulé discovered that benzene has a closed ring structure with alternating double bonds in 1865. The ring structure explained benzene's unusual stability and chemical properties. Prior to this, chemists had proposed linear or open-chain structures. Benzene's six carbon atoms form a hexagonal ring, which is why 'Closed Ring' is the correct answer over squares, rectangles, or boxes.