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"

pure water does not conduct electricity very well. however, when certain substances are dissolved in water, the solution does conduct electricity. you can make a simple device that shows how well a solution conducts electricity. this device uses a flashlight bulb to indicate how well the solution conducts electricity. the better the solution conducts electricity, the brighter the bulb will glow.

explanation:

the correct answer to this question is 2