Which of the following structures is the smallest

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If we refer to smallest structures to volume occupyingentities in nature, then the smallest known will have to be mesons (quark and anti-quark pair, like the pion $pi$), whose volume and shape is still a matter of research, but you can take an upper bound to be of order of the size of the nucleus of Hydrogen (a proton): $10^{-12}$ cm.

Explanation:

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