Option 1: This option states that both statements I and II are individually true and that statement II is the correct explanation of statement I. Let`s analyze each statement separately:

- Statement I: Limestone decomposes when it is heated in air. This statement is true. Limestone undergoes thermal decomposition and releases carbon dioxide gas when heated in air.

- Statement II: Increase in the content of CO2 in the atmosphere in recent years is mainly due to the use of limestone in the manufacture of cement. This statement implies that the increase in CO2 levels is mainly caused by using limestone in cement production. However, this is incorrect. While limestone does contribute to CO2 emissions during cement production, it is not the main reason for the increase in atmospheric CO2 levels. The primary factor driving the increase in CO2 levels is the burning of fossil fuels.

Option 2: This option states that both statements I and II are individually true, but statement II is not the correct explanation of statement I. This is the correct answer. As mentioned before, statement I is true, but statement II is not the correct explanation for it.

Option 3: This option states that statement I is true, but statement II is false. However, as explained earlier, both statements I and