The mode of expression in which the concentration of a solution remains independent of temperature is Option D: Molality.

Molality is defined as the number of moles of solute per kilogram of solvent. Its formula is given by:

$$ molality = \frac{moles \; of \; solute}{kilograms \; of \; solvent} $$

Because molality involves mass (kilograms of solvent), it does not depend on temperature. Mass does not change with temperature, unlike volume, which can expand or contract with temperature changes. Hence, molality remains constant regardless of temperature changes.

For the sake of completeness, here's why the other options listed are temperature-dependent:

Molarity (Option A) is defined as moles of solute per liter of solution. Since volume can change with temperature, molarity is temperature-dependent.

Normality (Option B) depends on the equivalent concept which is context-sensitive to the reaction occurring. It typically depends on volume (like molarity) for solutions, so it can vary with temperature due to volume changes.

Formality (Option C) is similar to molarity but refers to moles of solute per liter of solution for substances that do not dissociate or react in solution. While it can be useful for non-reactive solutes, it still depends on the volume of the solution, making it temperature-dependent.