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!

I believe the correct answer from the choices listed above is option A. The statement that is true about a redox reaction would be that an electron is lost by the reducing agent. A reducing agent is oxidized, because it loses electrons in the redox reaction.  Hope this answers the question.