We need to match each complex with:

• Primary valency = oxidation state of the central metal ion
• Secondary valency = coordination number of the central metal ion

So for each complex, we find:

1. Oxidation state of the metal
2. Number of donor atoms attached to the metal

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1. Complex (A): $[\mathrm{Co(en)_2Cl_2}]\mathrm{Cl}$

Step 1: Find oxidation state of Co

The outer $\mathrm{Cl^-}$ means the complex ion has charge $+1$:

$[\mathrm{Co(en)_2Cl_2}]^+$

Let oxidation state of Co be $x$.

• $\mathrm{en}$ is neutral
• 2 coordinated $\mathrm{Cl^-}$ contribute $-2$

So,

$x + 0 - 2 = +1$ $x=+3$

Thus, primary valency = 3.

Step 2: Find coordination number

• Each $\mathrm{en}$ is bidentate: $2 \times 2 = 4$
• Two $\mathrm{Cl}$ ligands are monodentate: $2$

Total coordination number:

$4+2=6$

Thus, secondary valency = 6.

So,

$(A) \to (3,6) = (I)$

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2. Complex (B): $[\mathrm{Pt(NH_3)_2Cl(NO_2)}]$

Step 1: Find oxidation state of Pt

This complex is neutral.

• $\mathrm{NH_3}$ is neutral
• $\mathrm{Cl^-}$ is $-1$
• $\mathrm{NO_2^-}$ is $-1$

Let oxidation state of Pt be $x$:

$x+0-1-1=0$ $x=+2$

Thus, primary valency = 2.

Step 2: Find coordination number

Ligands present:

• $2$ ammine ligands: $2$
• $1$ chloro ligand: $1$
• $1$ nitro/nitrito ligand: $1$

Total coordination number:

$2+1+1=4$

Thus, secondary valency = 4.

So,

$(B) \to (2,4) = (IV)$

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3. Complex (C): $\mathrm{Hg[Co(SCN)_4]}$

Treat it as $\mathrm{Hg^{2+}}$ and $[\mathrm{Co(SCN)_4}]^{2-}$.

Step 1: Find oxidation state of Co

Let oxidation state of Co be $x$. Each $\mathrm{SCN^-}$ has charge $-1$.

$x + 4(-1) = -2$ $x-4=-2$ $x=+2$

Thus, primary valency = 2.

Step 2: Find coordination number

There are four $\mathrm{SCN^-}$ ligands, each monodentate.

So coordination number = 4.

Thus, secondary valency = 4.

So,

$(C) \to (2,4) = (IV)$

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4. Complex (D): $[\mathrm{Mg(EDTA)}]^{2-}$

Step 1: Find oxidation state of Mg

EDTA ligand is $\mathrm{EDTA^{4-}}$. Let oxidation state of Mg be $x$:

$x + (-4) = -2$ $x=+2$

Thus, primary valency = 2.

Step 2: Find coordination number

EDTA is a hexadentate ligand.

So coordination number = 6.

Thus, secondary valency = 6.

So,

$(D) \to (2,6) = (III)$

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5. Final matching

We get:

• $(A) \to (I)$
• $(B) \to (IV)$
• $(C) \to (IV)$
• $(D) \to (III)$

Now compare with options:

• Option A: $(B)$ wrong, $(C)$ wrong, $(D)$ wrong
• Option B: $(A)$ wrong
• Option C: $(A)-(I), (B)-(IV), (C)-(II), (D)-(III)$
• Option D: wrong

Here, everything matches Option C except complex (C).

For $\mathrm{Hg[Co(SCN)_4]}$, the chemically correct values are:

$\text{Primary valency} = 2, \quad \text{Secondary valency} = 4$

So $(C)$ should match (IV), not (II).

Thus the consistent full matching should be:

• $(A)-(I)$
• $(B)-(IV)$
• $(C)-(IV)$
• $(D)-(III)$

Since no option gives this, the stored answer appears inconsistent with standard coordination chemistry definitions.

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Conclusion

Using standard definitions, the given options do not contain the fully correct matching. The stored answer C seems incorrect because $(C)$ has been mismatched.