Step 1: Identify the deciding factor.
A concentration term stays constant with temperature only if none of its quantities depend on temperature.
Step 2: Test each term.
Molarity and normality are defined per litre of solution; volume expands on heating, so both fall slightly as temperature rises. Molality is defined per kilogram of solvent, and mass is a temperature-independent quantity.
Step 3: Conclude.
Only molality is unaffected by temperature, so the answer is option (i).
\[\boxed{\text{Molality}}\]