The heterocyclic compound below is a remarkably stable carbene often used as a ligand in organometallic chemistry. Explain the stability of this particular carbene.

The nitrogen in the ring makes the carbene carbon a triplet.

The double bond in the ring causes the carbene carbon to gain two more electrons.

The nitrogen in the ring stabilizes the carbene carbon inductively and mesomerically.

The double bond in the ring causes the carbene carbon to lower the energy of its HOMO.