Answer: Five and Six
Explanation: In nature, usually we can observe the rings with 5 or 6 carbon atoms are most abundant.
First of all, the rings with 5 or 6 carbon atoms are the most stable rings which are not affected by any free energy factor or enthalpy factor or entropy factor.
Moreover 6 membered rings are found to be the most stable ones because of the extra stability provided by the term known as Aromaticity like Benzne is known to be most stable and most aromatic compound.
5 membered rings are found in our body as ribose sugar which makes the important nucleic acids which are RNA (ribose nucleic acids)and DNA(de oxyribose nucleic acids) .
