For them following reactions, the δh° rxn is not equal to δh°f for the product except for

a) 3mg(s)+n2(g)—> mg3n2(s)
b) 2ca(s)+o2(g)—> 2cao(s)
c)c2h2(g)+h2(g)—> c2h4(g)
d) 2c(graphite)+o2(g)—> 2co(g)
e) c(diamond)+o2(g)—> co2(g)

The answer is (e)

Enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states.

(a) does not start with elements
(b) does not start with carbon in its standard state
(c) and (d) form TWO moles of the product

Only (e) complies with all parts of the definition.

What does that mean you dum bum