For which of the following pairs of temperature scales is the size...

The size of a degree on the Celsius scale is defined to be exactly equal to the size of a degree on the absolute (Kelvin) scale. A temperature difference of $1^{{}^{{}^{\circ}}}\text{C}$ is equivalent to a temperature difference of $1\text{ K}$.

The ideal gas temperature scale is based on the properties of an ideal gas and is theoretically identical to the absolute thermodynamic temperature scale (Kelvin scale). Therefore, the size of a degree on the ideal gas scale is identical to that on the absolute scale.

Since the Celsius, ideal gas, and absolute scales all share the same degree size, the pairs (Celsius scale and ideal gas scale) and (ideal gas scale and absolute scale) have identical degree sizes.

The mercury scale is an empirical scale based on the volumetric expansion of mercury. Because the coefficient of expansion of mercury is not perfectly constant with temperature, the size of a degree on the mercury scale is not strictly identical to the thermodynamic or ideal gas scales at all temperatures.