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

Let`s analyze each statement:

Statement I: Ozone is a tn-atomic molecule of oxygen.

This statement is true. Ozone is a type of molecule that is made up of three oxygen atoms. Its chemical formula is O3. This makes it different from the ordinary diatomic oxygen molecule (O2).

Statement II: Ozone is concentrated mainly in the stratosphere.

This statement is also true. The stratosphere is a layer of Earth`s atmosphere located above the troposphere. It extends from about 10 kilometers (6 miles) to 50 kilometers (31 miles) above the Earth`s surface. Ozone is indeed concentrated in the stratosphere, particularly in a region known as the ozone layer.

However, Statement II is not the correct explanation of Statement I. The fact that ozone is concentrated in the stratosphere does not explain why it is a trivalent (tn-atomic) molecule. The trivalent nature of ozone is due to the way the three oxygen atoms are bonded together, which is different from the way the two oxygen atoms are bonded in diatomic oxygen.

Therefore, the correct answer is option