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

Concept :

• For bond breaking heat is required.
• For bond-forming energy generated.
• Net energy change is the enthalpy of formation.
• Enthalpy of energy is the energy change due to the formation of one mole of the substance.
• Exothermic reactions are those reactions for which energy is generated.
• For exothermic reaction enthalpy of formation is negative.
• Endothermic reactions are those for which energy is absorbed.
• For endothermic reaction enthalpy of formation is positive.

Explanation:

• A reaction may be exothermic or endothermic. 
• If the energy released due to bond formation is greater than the energy required for bond breaking then the reaction is exothermic.
• For exothermic reaction enthalpy of formation is positive.
• If the energy released due to bond formation is less than the energy required for bond breaking then the reaction is endothermic.
• For endothermic reaction enthalpy of formation is positive.

Conclusion :

As the reaction may be endothermic or exothermic 

Then the enthalpy of formation of the reaction may be positive and negative 

So, the correct option is 3.