The first bond which forms between two atoms is known as Sigma Bond. It is considered as the strongest covalent bond which forms by the head-on overlapping between two orbitals.
In Benzene there are six sp² hybridized carbon atoms. Each carbon atom is bonded to two carbons atoms and one hydrogen atom. Each carbon atom is bonded to one carbon atom via a double bond and through single bond to another carbon atom. Hence, there are Six sigma bonds between carbon atoms in Benzene.
In Cyclobutane there are four sp³ hybridized carbon atoms. Each carbon atom is singly bonded to two other carbon atoms and two hydrogen atoms. Therefore, there are FOUR sigma bonds between the carbon atoms of cyclobutane.
The sigma bonds are highlighted BLUE in given structures,
