1. Identify the metal with least metallic radius

Among the given 3d-series elements: $\mathrm{Sc},\ Ti,\ V,\ Cr,\ Mn,\ Zn$ metallic radius generally decreases across a period from left to right due to increasing effective nuclear charge.

Hence, among these, the least metallic radius is for: $\boxed{\mathrm{Zn}}$

So the ion is: $\mathrm{ZnO_4^{2-}}$

2. Find the oxidation state of Zn

Let oxidation state of Zn be $x$.

Each oxygen has oxidation number $-2$.

So, $x + 4(-2) = -2$ $x - 8 = -2$ $x = +6$

Thus, Zn is in oxidation state: $\boxed{+6}$

3. Find the electronic configuration of Zn and $\mathrm{Zn^{6+}}$

Atomic number of Zn = 30

Ground-state electronic configuration of Zn: $\mathrm{Zn} : [Ar]3d^{10}4s^2$

For $\mathrm{Zn^{6+}}$, remove 2 electrons from $4s$ and 4 electrons from $3d$: $\mathrm{Zn^{6+}} : [Ar]3d^6$

4. Determine number of unpaired electrons in $\mathrm{ZnO_4^{2-}}$

$\mathrm{MO_4^{2-}}$ is a tetrahedral oxoanion. For a tetrahedral field, splitting is small and generally high spin.

So for $d^6$ tetrahedral configuration, the electron arrangement has 4 unpaired electrons.

Thus, $n = 4$

5. Calculate spin-only magnetic moment

Spin-only magnetic moment is: $\mu = \sqrt{n(n+2)}\ \text{BM}$

Substitute $n=4$: $\mu = \sqrt{4(4+2)} = \sqrt{24}$ $\mu \approx 4.90\ \text{BM}$

Since the question is of integer type, the required value is: $\boxed{5}$

6. Compare with stored correct answer

Stored correct answer = $0$

My derived answer is $5$, so I do not agree with the stored answer.

The likely reason is that the intended species may have been assumed differently, but based on the given wording, the metal with least metallic radius is Zn, leading to $\mathrm{ZnO_4^{2-}}$ and a nonzero magnetic moment.