The red color of ruby is due to a d-d transition of Cr3+ ion in Al2O3 lattice.

Ruby is a gemstone that is a form of the mineral corundum (Al2O3) and is valued for its rich red color. The red color of ruby is primarily due to the presence of chromium (Cr3+) ions in the crystal lattice of corundum.

d-d transition of Cr3+ ion in Al2O3 lattice:
- The red color of ruby is a result of a specific electronic transition known as a d-d transition.
- In the crystal lattice of Al2O3, some of the aluminum (Al3+) ions are replaced by chromium (Cr3+) ions.
- The Cr3+ ions have three electrons in their d orbitals, and these electrons can absorb light in the visible region of the electromagnetic spectrum.
- The absorption of light by the Cr3+ ions causes an electronic transition from one d orbital to another d orbital of higher energy.
- This d-d transition corresponds to the absorption of light in the blue-green region of the spectrum, and the complementary color, which is transmitted or reflected, is red.
- Therefore, when white light passes through or is reflected by a ruby, the blue-green light is absorbed by the Cr3+ ions, and the remaining red light gives the gemstone its characteristic red color.

Other options:
- Option a) d-d transition of Cr3+ ion in Cr2O3 lattice: This option is incorrect because ruby is composed of Al2O3 lattice, not Cr2O3.
- Option c) Ligand to metal charge transfer transition: This option is incorrect because ligand to metal charge transfer transitions typically involve the transfer of an electron from a ligand to a metal ion, resulting in a different color range.
- Option d) Metal to metal charge transfer transition: This option is incorrect because metal to metal charge transfer transitions involve the transfer of an electron between two metal ions, which is not applicable to the chromium ions in ruby.

In conclusion, the red color of ruby is specifically due to a d-d transition of Cr3+ ions in the Al2O3 lattice, resulting in the absorption of blue-green light and the transmission or reflection of red light.