The correct answer for the question is option 3: Iron compounds. The red color of the soil is primarily caused by the presence of iron compounds. Iron is present in the soil in the form of iron oxides, particularly iron oxide called hematite. Hematite is responsible for the red color because it has a reddish-brown appearance.

Option 1: Aluminium compounds - Although aluminum compounds can be found in soil, they are not the primary cause of red soil color.

Option 2: Mercury compounds - Mercury compounds do not play a significant role in the coloration of soil.

Option 4: Clay - Although clay can contribute to the overall composition of the soil, it doesn`t directly cause the red color.

It is important to note that the color of soil can vary depending on the region and the specific composition of the soil. However, in general, red soil is commonly associated with iron compounds.