In the context of Indian physical geography, where are fertile alluvial soils predominantly found within India?

Introduction / Context:
This question is about the distribution of alluvial soils in India. Alluvial soils are among the most important and fertile soils in the country and support a large share of India's agricultural production. Understanding where these soils are predominantly found is a basic yet crucial point in physical and agricultural geography.

Given Data / Assumptions:
- The question asks about the main regions where alluvial soils occur in India.
- The options contrast northern plains and river valleys with mountains, plateaus, coasts and deserts.
- We assume the standard soil classification used in Indian geography textbooks.

Concept / Approach:
Alluvial soils are formed by the deposition of silt, sand and clay carried by rivers. In India, the largest and most continuous belt of alluvial soils is found in the Great Northern Plains formed by the Indus, Ganga and Brahmaputra river systems and their tributaries. These soils also occur in river valleys and deltas in peninsular India, but the core region is the extensive northern plains from Punjab and Haryana in the west to Assam and West Bengal in the east. Therefore, the correct approach is to associate alluvial soils mainly with the northern plains and major river valleys, not with high mountains, rocky plateaus or desert dunes.

Step-by-Step Solution:
Step 1: Recall the definition of alluvial soil as soil deposited by rivers, particularly in floodplains and deltas.Step 2: Identify the Great Northern Plains of India, stretching along the Indus, Ganga and Brahmaputra systems, as the main zone where these deposits accumulate.Step 3: Consider that other regions, such as interior plateaus or high mountains, are more associated with black soil, red soil or mountain soil rather than thick alluvial deposits.Step 4: Examine the options: the only one that correctly describes the northern plains and major river valleys as the dominant alluvial regions is option (a).Step 5: Coastal sandy belts and desert dunes are mainly depositional environments of wind or sea waves, not classic riverine alluvium.Step 6: Therefore, choose option (a) as the correct description of where alluvial soils are mainly found in India.

Verification / Alternative check:
Soil maps of India clearly mark the Indo-Gangetic-Brahmaputra plains as the largest continuous belt of alluvial soils. Textbooks often highlight that this region is the food-grain basket of the country because of the deep, fertile alluvium. River valleys and deltas in peninsular India also show patches of alluvial soil, but they are smaller in comparison. No authoritative map shows alluvial soils as dominant in high mountain ranges, rocky plateaus or desert dunes, which confirms that the northern plains and river valleys are the primary regions.

Why Other Options Are Wrong:
Only in the high Himalayan and Karakoram mountain ranges: These regions mainly have thin, immature mountain soils and exposed rock rather than extensive alluvial plains.

Exclusively in the interior rocky plateaus of peninsular India: Plateaus are more associated with black, red and lateritic soils than with deep alluvium.
Restricted only to narrow sandy belts along the seacoasts: Coastal sand is mostly marine and aeolian in origin, not thick riverine alluvium.
Found mainly in desert dunes of western Rajasthan: Desert dunes are formed by wind-blown sand and are not typical zones of fertile alluvial soil.

Common Pitfalls:
Some learners misunderstand the term "alluvial" and imagine any loose or sandy soil, including deserts and coasts, to be alluvial. Another pitfall is overlooking the word "predominantly" and thinking only of small alluvial patches near plateau rivers. A reliable strategy is to visualise the broad green belt of the Indo-Gangetic-Brahmaputra plains on a map and remember that this region is dominated by alluvial deposits from major rivers.

Final Answer:
In the northern plains and major river valleys, especially along the Indus, Ganga and Brahmaputra systems