The correct answer is D. Highly variable.
The pH of red soils in Karnataka is highly variable, ranging from 4.5 to 8.5. This is due to the fact that red soils are formed from a variety of parent materials, including granite, gneiss, and basalt. The pH of these parent materials can vary depending on their composition and the weathering processes that have affected them.
In addition, the pH of red soils can be affected by the presence of organic matter. Organic matter can buffer the pH of soil, making it less susceptible to changes in the pH of the surrounding environment.
As a result of the factors mentioned above, the pH of red soils in Karnataka can vary widely. This can have a significant impact on the growth of plants, as different plants prefer different pH levels. For example, some plants, such as blueberries, prefer acidic soils, while others, such as rhododendrons, prefer alkaline soils.
It is important to test the pH of soil before planting any plants. This will help to ensure that the plants are growing in the correct pH environment.