Explanation:
The Jahn-Teller effect is observed in octahedral complexes with d-electron configuration of d5 and d6. Let's discuss each option in detail.

a) d5-high spin:
In d5-high spin complexes, the electrons are distributed in the t2g and eg orbitals in an octahedral field. As all the five d-electrons are unpaired, they occupy the t2g orbitals, and the eg orbitals remain empty. In this case, there is no distortion of the octahedral geometry due to the Jahn-Teller effect.

b) d5-low spin:
In d5-low spin complexes, the electrons occupy the t2g orbitals, and the eg orbitals remain empty due to the strong ligand field. As there are no electrons in the eg orbitals, the octahedral geometry is distorted due to the Jahn-Teller effect. The distortion occurs in such a way that the degeneracy of the t2g orbitals is lifted, and the energy levels split into two different levels.

c) d6-high spin:
In d6-high spin complexes, all six electrons occupy the t2g and eg orbitals in an octahedral field. As all the d-electrons are unpaired, they occupy the t2g and eg orbitals in such a way that the energy levels are degenerate. In this case, there is no distortion of the octahedral geometry due to the Jahn-Teller effect.

d) d6-low spin:
In d6-low spin complexes, the electrons fill the t2g orbitals first, and then the eg orbitals. As there are four electrons in the t2g orbitals and two electrons in the eg orbitals, the octahedral geometry is distorted due to the Jahn-Teller effect. The distortion occurs in such a way that the degeneracy of the t2g and eg orbitals is lifted, and the energy levels split into different levels.

Conclusion:
Therefore, the Jahn-Teller effect is observed in octahedral complexes with d-electron configuration of d5-low spin and d6-low spin.