The Isotherm Line mentioned in the question is a term used in meteorology to represent a line on a weather map that connects places with the same temperature. In the month of January, this line divides India into two nearly equal parts.

Option 1: 10 ºC - This option suggests that the isotherm line is at a temperature of 10 ºC. However, this is incorrect as the correct answer is given as option 4.

Option 2: 25 ºC - This option suggests that the isotherm line is at a temperature of 25 ºC. However, this is incorrect as the correct answer is given as option 4.

Option 3: 15 ºC - This option suggests that the isotherm line is at a temperature of 15 ºC. However, this is incorrect as the correct answer is given as option 4.

Option 4: 20 ºC - This option is the correct answer. It states that the isotherm line dividing India into two equal parts in January is at a temperature of 20 ºC.

In conclusion, the correct answer is option 4, which states that the isotherm line dividing India in January is at a