Spin paired complex ions are those in which all the electrons are paired up in the same direction, resulting in a diamagnetic complex. Diamagnetic complexes are not attracted to a magnetic field and have a low magnetic moment.

The correct answer is option 'D', [Co(NH3)6]3.

Explanation:
- Coordination number of complex ion = 6
- Oxidation state of central metal ion = +3
- Cobalt has 27 electrons, so [Co(NH3)6]3+ has 24 electrons
- NH3 is a strong field ligand, so it will pair up the electrons in the t2g orbitals before occupying the eg orbitals
- Therefore, all the electrons in the complex ion are paired up in the same direction, resulting in a diamagnetic complex
- Hence, [Co(NH3)6]3+ is a spin paired complex ion

Let's look at the other options:
a) [Cr(NH3)6]3+: Chromium has a d4 configuration, so it has one unpaired electron in the t2g orbitals. Hence, it is a paramagnetic complex.
b) [Cr(CN)6]3-: Cyanide is a strong field ligand, so it will pair up the electrons in the t2g orbitals before occupying the eg orbitals. However, chromium has a d3 configuration, so it will have one unpaired electron in the eg orbitals. Hence, it is a paramagnetic complex.
c) [Fe(C2O4)3]3-: Iron has a d5 configuration, so it has one unpaired electron in the t2g orbitals. Hence, it is a paramagnetic complex.