In which of the following pairs of temperature scales, the size of a degree is identical?
A) Mercury scale and ideal gas scale
B) Celsius scale and mercury scale
C) Celsius scale and ideal gas scale
D) Ideal gas scale and absolute scale

Celsius scale is developed based on freezing point (0 degree Celsius) and boiling point(100 degree Celsius) of water at standard atmosphere. The scale is divided by 100 equal parts between these points.

Ideal gas scale is based on ideal gas law. The pressure of an ideal gas varies linearly with temperature if the volume of the gas is held constant. By extrapolating P−T graph as P approaches to zero, T is also zero which is called absolute zero of temperature.

Absolute scale: It is developed based on fundamental laws of thermodynamics. A Carnot engine has 100% efficiency if we maintain the sink temperature at absolute zero. It is also known as thermodynamic scale of temperature.
Both ideal gas scale and absolute scale equal numerically at every point. 
These two are related to Celsius scale by 
T=t degree C +273.15
The size of a degree is identical in the above three scales
Options (C) and (D) are correct answer.