Climatological design period: The rainfall cycle period commonly adopted for India (for hydrologic design and analysis) is taken as:

Introduction / Context:
For regional hydrologic assessments, a representative climatic period is selected to smooth short-term variability while capturing multi-decadal oscillations. In India, a conventional “rainfall cycle period” has been used in various planning documents and texts.

Given Data / Assumptions:

• Objective: choose a period long enough to capture interannual and decadal variability.
• Uniformity in statistics (mean, variance) over the period is desired for planning norms.
• Traditional practice informs the adopted cycle length.

Concept / Approach:
A cycle period on the order of several decades is favored to encompass monsoon variability modes. The widely cited value in Indian practice is 35 years, providing a balance between statistical stability and data availability.

Step-by-Step Solution:
Evaluate candidate periods 15–35 years.Adopt the conventional period used in Indian hydrologic design references.Select 35 years as the rainfall cycle period.

Verification / Alternative check:
Many regional analyses and academic references cite 35 years as a typical climatological cycle length for rainfall studies in India.

Why Other Options Are Wrong:
Shorter spans (15–30) may not sufficiently capture low-frequency monsoon variability and can bias design values.

Common Pitfalls:
Confusing design storm return period with climatic cycle length; these are different concepts.

Final Answer:
35 years.