The correct answer is: b) Shallow depth and stoniness.
Mountain soils in Arunachal Pradesh are characterized by shallow depth and stoniness. This is because the mountains are young and the soil has not had time to develop a deep profile. The soil is also very stony, which makes it difficult to cultivate.
The other options are incorrect. Option a) is incorrect because mountain soils in Arunachal Pradesh are not typically very fertile. This is because the soil is often thin and rocky, and it can be difficult for plants to get the nutrients they need. Option c) is incorrect because mountain soils in Arunachal Pradesh do not typically have good water retention capacity. This is because the soil is often sandy or rocky, and it does not hold water well. Option d) is incorrect because mountain soils in Arunachal Pradesh do not typically have a neutral pH. The pH of the soil can vary depending on the type of rock that it is derived from, but it is often acidic.