Which of the following factors most strongly affects the rate of humus formation in soil?

Introduction:
Humus is the dark, organic component of soil that forms from the decomposition of plant and animal remains. This question checks understanding of the main environmental factors that control how quickly humus forms in a given region. It belongs to basic environmental science and geography, which are frequently tested in general knowledge examinations.

Given Data / Assumptions:

• Humus formation takes place through decomposition of organic matter in soil.
• The given options list possible controlling factors such as organisms, time, temperature, and parent rock.
• The question asks which factor most strongly affects the rate of humus formation.

Concept / Approach:
The rate of humus formation depends mainly on climate, especially temperature and moisture. Warm and moist conditions speed up the activity of decomposers such as bacteria, fungi, and other soil organisms. Cold or very dry conditions slow down decomposition, so humus accumulates very slowly. Other factors like flora, fauna, parent rock, and time do influence soil development, but the dominant control on the rate of humus formation is the prevailing climate.

Step-by-Step Solution:
Step 1: Recall that humus forms through biological decay of organic matter in soil.Step 2: Decomposition is a biochemical process that speeds up when temperature is moderate to high and moisture is adequate.Step 3: Climate, through temperature and rainfall, therefore controls how active decomposers are.Step 4: Flora, fauna, parent rock, and time all play roles, but they cannot override an unfavourable climate.Step 5: Among the options, temperature and climate of the region best represent this key control factor.

Verification / Alternative check:
In cold polar or high mountain regions, plant growth is slow and low temperatures reduce microbial activity, so very little humus forms even if organic matter is present. In contrast, in tropical regions with warm temperatures and good rainfall, organic matter decomposes quickly and large amounts of humus can form. This contrast confirms that temperature and climate are the main controls on the rate of humus formation.

Why Other Options Are Wrong:
Flora and fauna living in the soil influence the amount and type of organic matter, but climate still sets the overall pace of decomposition. Time taken for soil to form affects total soil depth, not the immediate rate of humus production. Nature of the parent rock only influences mineral composition and texture, not directly the rate of organic decay. Colour and appearance of the soil surface are results of processes rather than a controlling factor.

Common Pitfalls:
A common mistake is to assume that the presence of plants alone determines humus formation, without realising that even abundant vegetation decomposes slowly in very cold or dry climates. Another error is to confuse soil formation in general with humus formation, which is specifically the organic part of soil. Students should remember that climate, through temperature and moisture, is a basic control in almost all soil processes.

Final Answer:
Correct option: Temperature and climate of the region