Answer:

Diamagnetic ions are those ions that have all their electrons paired up in their atomic or molecular orbitals, resulting in no unpaired electrons. As a result, diamagnetic ions do not possess a net magnetic moment and are not attracted to a magnetic field.

Let's examine each option to determine which one is a diamagnetic ion:

a) CO2:
CO2 is a neutral molecule composed of carbon and oxygen atoms. Carbon has an atomic number of 6, and its electron configuration is 1s2 2s2 2p2. Oxygen has an atomic number of 8, and its electron configuration is 1s2 2s2 2p4. When the two carbon atoms and two oxygen atoms combine in CO2, the molecular orbitals are formed by the combination of atomic orbitals. The molecular orbital diagram for CO2 shows that there are two unpaired electrons in the π* antibonding orbitals. Therefore, CO2 is not a diamagnetic ion.

b) Cu2:
Cu2 refers to the copper ion with a +2 charge. Copper has an atomic number of 29, and its electron configuration is [Ar] 3d10 4s1. When copper loses two electrons to form Cu2+, it results in the electron configuration of [Ar] 3d9. The 3d orbital of copper has one unpaired electron, making Cu2 a paramagnetic ion. Therefore, Cu2 is not a diamagnetic ion.

c) Mn2:
Mn2 refers to the manganese ion with a +2 charge. Manganese has an atomic number of 25, and its electron configuration is [Ar] 3d5 4s2. When manganese loses two electrons to form Mn2+, it results in the electron configuration of [Ar] 3d5. The 3d orbital of manganese has five unpaired electrons, making Mn2 a paramagnetic ion. Therefore, Mn2 is not a diamagnetic ion.

d) Sc3:
Sc3 refers to the scandium ion with a +3 charge. Scandium has an atomic number of 21, and its electron configuration is [Ar] 3d1 4s2. When scandium loses three electrons to form Sc3+, it results in the electron configuration of [Ar] 3d0. The 3d orbital of scandium has no unpaired electrons, meaning that all its electrons are paired up. As a result, Sc3 is a diamagnetic ion.

Therefore, the correct answer is option 'D' - Sc3.