p-Amino-N,N-dimethylaniline is added to a strongly acidic solution of X. The resulting solution is treated with a few drops of aqueous solution of Y to yield blue colouration due to the formation of methylene blue. Treatment of aqueous solution of Y with reagent potassium hexacyanoferrate (II) leads to the formation of an intense blue precipitate. The precipitate dissolves on excess addition of the reagent. Similarly, the treatment of the solution of Y with the solution of potassium hexacyanoferrate (III) leads to a brown colouration due to the formation of Z.
Q. Compound X is

Options

A. `NaNO_3`
B. `NaCl`
C. `Na_2SO_4`
D. `Na_2S`

Explanation

To solve the question, we need to analyze the information provided step by step.

### Step 1: Identify the structure of p-Amino-N,N-dimethylaniline
The first step is to understand the structure of p-Amino-N,N-dimethylaniline. It has an amino group (-NH2) and two methyl groups (-CH3) attached to the nitrogen atom, and it is para-substituted on the benzene ring.

### Step 2: Reaction with a strongly acidic solution (Compound X)
When p-Amino-N,N-dimethylaniline is added to a strongly acidic solution (let's denote it as X), it is likely protonated, which makes it more reactive. The acidic environment is necessary for the formation of methylene blue.

### Step 3: Formation of Methylene Blue
The resulting solution is treated with an aqueous solution of Y, which contains Fe³⁺ ions. The reaction between p-Amino-N,N-dimethylaniline and Fe³⁺ leads to the formation of methylene blue, which is a blue-colored compound.

### Step 4: Treatment of Y with Potassium Hexacyanoferrate (II)
Next, we treat the aqueous solution of Y with potassium hexacyanoferrate (II) [K4[Fe(CN)6]]. This leads to the formation of an intense blue precipitate. The precipitate is likely Prussian blue (Fe₃[Fe(CN)₆]₂), which is known to form from the reaction of Fe²⁺ with Fe³⁺ in the presence of cyanide ions.

### Step 5: Treatment of Y with Potassium Hexacyanoferrate (III)
When the solution of Y is treated with potassium hexacyanoferrate (III) [K3[Fe(CN)6]], it results in a brown coloration due to the formation of a brown precipitate, which is likely a complex of iron with cyanide ions.

### Step 6: Identify Compound X
From the information given, we need to identify what compound X is. The only options provided are sodium salts, and since we are looking for a strongly acidic solution, the most suitable choice is sodium sulfide (Na₂S). Sodium sulfide in solution can release sulfide ions (S²⁻), which can create an acidic environment when reacting with the amine.

### Conclusion
Thus, the compound X is **Na₂S**.