The standard free energies represent the total available work required for the process. It also indicates whether a process is spontaneous or not. If the standard free energy is negative, then it is spontaneous. All the reactions given have positive free energies. So, the lowest value is then chosen. The answer is:
C. 2C(s)+2H2(g)?C2H4(g); ?G?f=68.20 kJ/mol