The specific heat is, by the definition, the quantity of heat that is required to raise the temperature of a mass unit of a substance by one degree. One may use any units of heat (energy), mass and temperature, but the numerical value of the specific heat will depend on this choice.
The specific heat is a characteristics of a substance itself, it does not depend on the mass of a body and the change of the temperature. But it is important that the state of aggregation remains the same.
In our case, the given temperatures are inside the interval where ice remains solid. Therefore the amount of heat is `C*m*Delta t = 0.492*100*(-0.5-(-20)) approx 959.4` (calories). This is the answer. Note that the all data are given in the suitable units (degrees, grams, calories).