For the reaction : `PCl_(5) (g) rarrPCl_(3) (g) +Cl_(2)(g)`:

Options

A. (a) Equal volumes of `PCl_(5),PCl_(3)andCl_(2)` are present.
B. (b) Equal masses of `PCl_(5),PCl_(3)andCl_(2)` are present.
C. (c) The concentrations of `PCl_(5),PCl_(3)andCl_(2)` become constant.
D. (d) Reaction stops

Explanation

To solve the question regarding the reaction \( \text{PCl}_5 (g) \rightleftharpoons \text{PCl}_3 (g) + \text{Cl}_2 (g) \), we need to analyze the options provided and determine which statement is correct concerning the equilibrium state of the reaction.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The given reaction is a reversible reaction where phosphorus pentachloride (\( \text{PCl}_5 \)) decomposes into phosphorus trichloride (\( \text{PCl}_3 \)) and chlorine gas (\( \text{Cl}_2 \)).

2. **Equilibrium Concept**:
In a reversible reaction, the forward and reverse reactions occur simultaneously. At equilibrium, the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products.

3. **Analyzing the Options**:
- **Option A**: States that equal volumes of \( \text{PCl}_5 \), \( \text{PCl}_3 \), and \( \text{Cl}_2 \) are present. This is incorrect because the volumes of gases do not have to be equal at equilibrium; they depend on the stoichiometry of the reaction and the initial amounts.

- **Option B**: Claims that equal masses of the gases are present. This is also incorrect. The masses of the gases will vary based on their molar masses and the amounts present at equilibrium.

- **Option C**: States that the concentrations of \( \text{PCl}_5 \), \( \text{PCl}_3 \), and \( \text{Cl}_2 \) become constant at equilibrium. This is correct because, at equilibrium, the concentrations of all species remain constant, although they may not be equal.

- **Option D**: Suggests that the reaction stops at equilibrium. This is incorrect. At equilibrium, the reaction continues to occur in both directions, but the concentrations remain constant, indicating a dynamic equilibrium.

4. **Conclusion**:
The correct statement regarding the equilibrium of the reaction is **Option C**, which states that the concentrations of \( \text{PCl}_5 \), \( \text{PCl}_3 \), and \( \text{Cl}_2 \) become constant.