The fluidity of molten metal of cast alloys (without any addition of fluxes) increases with increase in

The correct answer is degree of superheat.

Explanation:
Fluidity is the ability of a molten metal or alloy to flow readily into a mold cavity and fill all its intricate details before solidifying.

Let us evaluate how the options affect the fluidity of molten cast alloys:

1. Degree of Superheat:
The degree of superheat is defined as the difference between the pouring temperature of the molten metal and its liquidus temperature (the temperature at which solidification starts):
$\mathrm{Degree\; of\; Superheat}=T_{\mathrm{pouring}}-T_{\mathrm{liquidus}}$
Increasing the degree of superheat increases the sensible heat of the molten metal. This extra thermal energy delays the onset of solidification, keeping the metal liquid for a longer duration as it flows. Therefore, a higher degree of superheat directly increases the fluidity of the molten metal.

2. Viscosity:
Viscosity is the internal resistance of a fluid to flow. As the viscosity of molten metal increases, its flow resistance increases, which leads to a decrease in fluidity.

3. Surface Tension:
High surface tension makes it difficult for the liquid metal to wet the mold walls and flow into narrow channels or thin sections, thereby decreasing its fluidity.

4. Freezing Range:
The freezing range is the temperature gap between the liquidus and solidus lines. A wider freezing range promotes the formation of a mushy zone (solid and liquid mixed phase) early in the cooling process. The solid dendrites in the mushy zone restrict liquid flow, which significantly decreases fluidity.

Consequently, the fluidity of the molten metal increases with an increase in the degree of superheat.