The enthalpy of elements in their standard states are taken as zero. Hence the enthalpy of formation of a compound is 

Suppose we have two compounds A and B. And on combining both, the compound formed is AB. 

If the compound formed is stable then energy is released which is denoted as +ve and if the compound formed is unstable, then energy is absorbed which is denoted as -ve.

Hence, the enthalpy of the compound formed in standard states may be positive, maybe negative or zero.

Therefore, option (C) is correct.