The Brainliest Answer!
First it was atom but it had smaller atomic particles protons,netrons or electron. But, it was the quarks who made them.They should be the smallest but...Then now it is believed that These quarks are now made up of even smaller particles called string...Now, we dont know if its true or not Beacause it is just a theory 'String theory'.They may be also made up of even smaller particles. So, we are not sure about the answer...
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the answer is quarks
But now it is siad that quarks are indivisible particles and strings are those things who govern the quarks(For extra information)
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By theories and studies, Atom is the smaller but neutron, proton and electron are th SMALLEST particle