Multiple choice I genuinely have no idea-- but I don't think it's the first one or the last.
I think it may be the third one but I'm not too sure.

The correct option is;

By first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of reaction 2

Explanation:

The given reactions are;

1) C₂H₂ (g) +  2H₂ (g)  → C₂H₆ (g)          ΔH = -75 kcal

2) C₂H₄ (g) +  H₂ (g)  → C₂H₆ (g)          ΔH = -33 kcal

3) C₂H₂ (g) +  H₂ (g)  → C₂H₄ (g)          ΔH = ?

By reversing reaction (2) and subtracting the reactants from the products we then have followed by adding the enthalpy of reaction 1 to the value of the enthalpy of the (reversed) reaction 2, we have;

C₂H₂ (g) + 2H₂ (g)  → C₂H₆ (g)          ΔH = -75 kcal

C₂H₆ (g) → C₂H₄ (g) + H₂ (g)             ΔH = 33kcal

Adding, and cancelling out the italicized terms which are repeated in the product and reactants, we get

C₂H₂ (g) + H₂ (g) → C₂H₄ (g)         ΔH = -75 kcal + 33 kcal = -42 kcal

Therefore, the correct option is "by first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of reaction 2"

alienation, transformation, consumption, and the relativity of truth.

you can choose any one of these, they are all important themes.

these themes reflect the distinct sensibilities of both the modernist and postmodern aesthetic movements. (from google, you should paraphrase)

hope this !

- melanie