Problem: Localized Bonding

Describe the makeup of the bond the arrow points to, using the valence-bond localized model.

Analysis

First identify the hybridization of each atom, and then determine which hybrid atomic orbitals on each atom overlap to make the bond.

C3 is connected to 3 other atoms (the ring carbons and a hydrogen) so is sp2. The unhybridized p orbital is used to make the π bond.
C4 is connected to 4 other atoms: the two ring carbons plus two hydrogens not shown explicitly. Therefore it is sp3 hybridized.
We can illustrate these in the 3-D model:
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Show the three sp2 orbitals on C-3.
Show the four sp3 orbitals on C-4.
Show the sp2 orbital involved in the C-C bond
Show the sp3 or bital involved in the C-C bond
Show the two orbitals overlapping to form the σ bond.

For completeness, here are the p orbitals making the π bond.
The 2-D picture showing this is: