The latest discovered state of matter is

The Bose-Einstein condensate (BEC) is considered the latest discovered state of matter among the options provided. While solids, liquids, and gases have been known since antiquity, plasma was identified in the late 19th and early 20th centuries. The BEC was theoretically predicted by Satyendra Nath Bose and Albert Einstein in the 1920s, but it was not experimentally realized until June 5, 1995. Eric Cornell and Carl Wieman achieved this by cooling rubidium atoms to 170 nanokelvins, near absolute zero, causing the atoms to behave as a single quantum entity. Although even newer states like Fermionic condensates were discovered in 2003, BEC remains the most recently discovered 'standard' fifth state of matter commonly cited in scientific literature and textbooks.