Why are pi bonds weaker than sigma bonds?

Pi bonds are usually weaker than sigma bonds. Quantum mechanics says this is because the orbital paths are parallel so there is much less overlap between the p-orbitals. 

It's basically depend on the formation of these bonds.

Hope this helps!

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Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.