Option 1: Both the statements are individually true and statement II is the correct explanation of statement I.

This option suggests that both statements I and II are true. Statement I states that the decay and disintegration of rocks in situ is called weathering, which is generally correct. Statement II states that mechanical weathering is mainly caused by temperature variation, which is not the most accurate explanation. While temperature variation can contribute to mechanical weathering, it is not the only factor. Mechanical weathering can also be caused by other processes such as freeze-thaw cycles, abrasion, and impact.

Option 3: Statement I is true but Statement II is false.

This option suggests that Statement I is true, which is accurate. However, it states that Statement II is false, which is not entirely correct. Although temperature variation is not the sole cause of mechanical weathering, it can still contribute to the process.

Option 4: Statement I is false but statement II is true.

This option suggests that Statement I is false, which is incorrect. The decay and disintegration of rocks in situ is indeed referred to as weathering. Statement II, which claims that mechanical weathering is mainly caused by temperature variation, is true but not the full explanation.

Based on the explanations above,