The correct statement is

Options

A. Green vitriol and blue vitriol are isomorphus
B. `KMnO_(4)`, and `K_(2)Cr_(2)O_(7)`are coloured due to d-d transitions
C. `Cu_(2)Cl_(2)` and `Ag_(2)S` are coloured
D. Upon strong heating paramagnetic gases are evolved by `NaNO_(3)`, and `AgNO_(3)`

Answer

To solve the question regarding which statement is correct, we will analyze each option step by step.

### Step 1: Analyze Statement 1
**Statement:** Green vitriol and blue vitriol are isomorphous.

- **Green vitriol** is FeSO4·7H2O (Iron(II) sulfate heptahydrate).
- **Blue vitriol** is CuSO4·5H2O (Copper(II) sulfate pentahydrate).

**Isomorphous substances** are those that have the same crystal structure and similar chemical composition. Since the chemical formulas of green vitriol and blue vitriol are different (FeSO4 vs. CuSO4), they are not isomorphous.

**Conclusion:** Statement 1 is **false**.

### Step 2: Analyze Statement 2
**Statement:** KMnO4 and K2Cr2O7 are colored due to d-d transitions.

- **KMnO4** contains Mn in the +7 oxidation state, which has the electronic configuration of [Ar] 3d0 4s0.
- **K2Cr2O7** contains Cr in the +6 oxidation state, which also has the electronic configuration of [Ar] 3d0 4s0.

Since both compounds have no d electrons (3d0), they cannot exhibit color due to d-d transitions. Instead, they are colored due to charge transfer spectra.

**Conclusion:** Statement 2 is **false**.

### Step 3: Analyze Statement 3
**Statement:** Cu2Cl2 and Ag2S are colored.

- **Cu2Cl2** (Copper(I) chloride) has Cu in the +1 oxidation state, with the electronic configuration of [Ar] 3d10 4s0.
- **Ag2S** (Silver sulfide) has Ag in the +1 oxidation state, with the electronic configuration of [Kr] 4d10 5s0.

Both Cu+ and Ag+ have completely filled d orbitals (3d10 and 4d10 respectively), which means they do not have unpaired electrons and thus do not exhibit color.

**Conclusion:** Statement 3 is **false**.

### Step 4: Analyze Statement 4
**Statement:** Upon strong heating, paramagnetic gases are evolved by NaNO3 and AgNO3.

- **NaNO3** decomposes upon heating to produce NaNO2 and O2 gas.
- **AgNO3** decomposes upon heating to produce Ag, NO2, and O2 gas.

O2 is a paramagnetic gas due to the presence of unpaired electrons.

**Conclusion:** Statement 4 is **true**.

### Final Answer
The correct statement is **Option 4**: Upon strong heating, paramagnetic gases are evolved by NaNO3 and AgNO3.

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