Introduction:
Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. In other words, these species have identical electron configurations, resulting in similar chemical and physical properties. However, not all combinations of atoms, ions, or molecules can be isoelectronic. This response will explain why option 'D' (Two different atoms) cannot be isoelectronic.

Explanation:
To understand why two different atoms cannot be isoelectronic, let's first define what it means for species to be isoelectronic.

Isoelectronic:
Isoelectronic species have the same number of electrons. This means that they have the same electron configuration, with the same number of electrons in each energy level or shell. For example, both neon (Ne) and sodium ion (Na+) are isoelectronic because they have the same electron configuration: 1s2 2s2 2p6.

Atoms:
Atoms are the fundamental building blocks of matter. Each atom is defined by the number of protons in its nucleus, which determines its atomic number. Different atoms have different numbers of protons, and therefore, different electron configurations.

Why two different atoms cannot be isoelectronic?
Different number of electrons:
Since different atoms have different numbers of protons, they also have different numbers of electrons. For example, carbon (C) has 6 electrons, while nitrogen (N) has 7 electrons. Therefore, carbon and nitrogen cannot be isoelectronic because they do not have the same number of electrons.

Different electron configurations:
Since different atoms have different numbers of electrons, they also have different electron configurations. Electron configuration refers to the arrangement of electrons in the energy levels or shells around the nucleus. For example, carbon has the electron configuration 1s2 2s2 2p2, while nitrogen has the electron configuration 1s2 2s2 2p3. These different electron configurations result in different chemical and physical properties for carbon and nitrogen.

Conclusion:
In conclusion, two different atoms cannot be isoelectronic because they have different numbers of electrons and different electron configurations. Isoelectronic species must have the same number of electrons and the same electron configuration.