Explanation:
When a diatomic molecule is irradiated with electromagnetic radiation, it absorbs energy and undergoes a rotational transition from one energy level to another. This results in the appearance of rotational lines in its spectrum. However, for a molecule to show rotational spectra, it must satisfy certain conditions. Let's see what these conditions are and then apply them to the given options.

Conditions for a molecule to show rotational spectra:

1. The molecule must be diatomic: This is because only diatomic molecules have a non-zero dipole moment and can interact with electromagnetic radiation.

2. The molecule must have a permanent dipole moment: This is necessary for the molecule to have a rotational energy level structure.

3. The molecule must not be in a vibrationally excited state: This is because a molecule in a vibrationally excited state will have a different rotational energy level structure.

Now, let's apply these conditions to the given options:

a) N2: Nitrogen is a diatomic molecule, but it has a zero dipole moment. Therefore, it cannot interact with electromagnetic radiation and will not show rotational spectra.

b) CO: Carbon monoxide is a diatomic molecule with a non-zero dipole moment. It will show rotational spectra.

c) NO: Nitric oxide is a diatomic molecule with a non-zero dipole moment. It will show rotational spectra.

d) HF: Hydrogen fluoride is a diatomic molecule with a non-zero dipole moment. It will show rotational spectra.

Therefore, the correct answer is option 'A' - N2, as it does not have a permanent dipole moment and cannot interact with electromagnetic radiation to show rotational spectra.