The standard heat of formation for cdo(s) is -258 kj/mol at 298 k. write the formation equation for cdo(s) that goes with this value of δh°. use the pull-down boxes to specify states. write fractions with a slash, such as 1/2 for one half. if a box is not needed leave it blank.

Cd(s) + 1/2 O₂(g) ⇒ CdO(s) ΔH° = -258 kJ/mol

Explanation:

The standard heat of formation corresponds to the reaction of synthesis of 1 mole of the desired product from the simple substances in their most stable form under standard conditions (1 bar, 25°C). The resulting thermochemical equation is:

Cd(s) + 1/2 O₂(g) ⇒ CdO(s) ΔH° = -258 kJ/mol

the answer your looking for is c).