Explanation:

Langmuir Isotherm Equation:
The Langmuir isotherm is a model that describes the adsorption of molecules onto a solid surface. The Langmuir isotherm equation is given by:
\[
\theta = \frac{K \cdot p}{1 + K \cdot p}
\]
where:
- \(\theta\) is the fractional coverage of the surface
- \(K\) is the Langmuir constant
- \(p\) is the pressure of the gas phase

Interpretation of Langmuir Isotherm Equation:
- The Langmuir isotherm equation represents a monolayer adsorption process where the adsorbate molecules form a single layer on the surface.
- As the pressure of the gas phase increases, the fractional coverage also increases, but it reaches a saturation point at higher pressures.

Correct Answer:
The correct form for a simple Langmuir isotherm is:
\(\theta = \frac{K \cdot p}{1 + K \cdot p}\)

Explanation of Correct Answer:
- The correct answer is option 'C', which is \(\theta = \frac{K \cdot p}{1 + K \cdot p}\).
- This equation represents the Langmuir isotherm in a simple form where the fractional coverage (\(\theta\)) is directly proportional to the pressure of the gas phase (\(p\)).
- The Langmuir constant (\(K\)) in the denominator accounts for the saturation behavior of the adsorption process.
In conclusion, the Langmuir isotherm equation is a fundamental model in surface chemistry that describes the adsorption behavior of molecules on a solid surface. The correct form of the Langmuir isotherm equation is crucial for understanding and predicting adsorption processes in various applications.