The correct answer is option 1: humus. Mountain soil is typically rich in humus, which is the organic component of soil formed from the decomposition of plant and animal matter. Humus helps to improve the fertility and structure of soil, making it more capable of retaining water and nutrients. This is especially important in mountainous areas, where the soil can be thinner and less fertile than in lowlands.

Option 2, clay, refers to a specific type of soil particle that can be found in varying amounts in different types of soil, including mountain soil. However, it is not the defining characteristic of mountain soil.

Option 3, coarse material, typically refers to larger particles such as rocks and gravel that can be found in mountain soil, but it is not the main component.

Option 4, iron and aluminum salts, may be present in mountain soil but they are not the primary component.

Overall, option 1 - humus - is the best answer that captures the important characteristic of mountain soil.