The Kelvin and Celsius scales are two of the most commonly used temperature scales in the physical sciences. The conversion from Kelvin to Celsius and vice versa is straightforward. On the Kelvin scale, 0 K is absolute zero, the coldest possible temperature, where all thermal motion stops. The main difference between Kelvin and Celsius is their starting points: 0 K is -273.15 degrees Celsius.

Option 1 is incorrect, subtracting the Celsius temperature from 273 would give a value lower than absolute zero which isn`t physically possible.

Option 3 and 4 use the value of 173 which isn`t the correct conversion rate between these two scales.

Option 2, which says x=273+y, is correct because it correctly states that to convert from Celsius to Kelvin, we have to add 273 to the Celsius value. This reflects the difference in starting points of the two scales.