Multiple Choice Question about Enthelpy of Formation

[TheExibo]

Homework Statement

The term chlorine is used to mean a number of possible types of matter containing chlorine atoms, such as Cl₂(g), Cl(g), and Cl^-(aq). Which of these forms of chlorine would have a zero standard enthalpy of formation?

a) Cl₂(g), Cl(g), and Cl^-(aq)

b) Cl(g)

c) Cl^-(aq)

d) Cl₂(g)

e) Cl₂(g), Cl(g)

Homework Equations

Enthalpy of Formation for Elements = 0KJ

The Attempt at a Solution

Really not sure where to begin, but the answer in the back of the book is d), and I can't understand why it's like that. Shouldn't it be a), since the enthalpy of formation for elements is always 0KJ

 

[Borek]

Can you quote the definition of a standard state as was given to you?

 

[brake4country]

What is the standard state of Chlorine? (Hint: standard state for pure liquids/gas is...)