Option 1: This option suggests that the enormous thickness of sedimentary rocks in Fold Mountains is due to the deposition of sediments in a valley for millions of years. However, this explanation is not accurate. Fold Mountains are formed by the tectonic forces that cause the Earth`s crust to buckle and fold. The deposition of sediments in a valley alone would not result in the formation of Fold Mountains.

Option 2: This is the correct answer. It states that the enormous thickness of sedimentary rocks in Fold Mountains is due to the accumulation of sediments in a geosyncline. A geosyncline is a trough or basin between two Fold Mountains where sedimentary rocks can accumulate over millions of years. As the tectonic plates move and Fold Mountains are formed, the sediments in the geosyncline get compressed and folded, leading to an increase in thickness.

Option 3: This option suggests that the Fold Mountains are formed by folding the plains. However, Fold Mountains are formed by the folding and uplift of the Earth`s crust, not by folding the plains.

Option 4: This option suggests that the sediments in Fold Mountains are folded into recumbent and nappe folds. While folding is a characteristic feature of Fold Mountains, rec