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

Explanation: This option suggests that both statements are true and that Statement II provides the correct explanation for Statement I.

Option 2: Both the statements are individually true but Statement II is not the correct explanation of Statement I.

Explanation: This option agrees that both statements are true, but it suggests that Statement II does not provide the correct explanation for Statement I.

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

Explanation: This option suggests that Statement I is true but Statement II is false.

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

Explanation: This option suggests that Statement I is false but Statement II is true.

In this particular case, Option 2 is the correct answer. Both statements are individually true, meaning that the decrease of air temperature with increasing altitudes is indeed called the vertical temperature gradient (Statement I), and in the troposphere, the air temperature decreases with increasing altitude due to radiation from the Earth (Statement II). However, Statement II is not the correct explanation of Statement I, as the vertical temperature gradient can also be affected by other factors such as pressure and humidity.