The right answer is B) NADH and FADH2.
NADH, FADH2 are molecules involved in the Krebs cycle, they carry electrons and H + protons.
NAD is involved in metabolism as an electron carrier in oxidation-reduction reactions, NAD + as an oxidant and NADH as a reducing agent.
Flavin adenine dinucleotide (FAD) is an oxidation-reduction cofactor derived from riboflavin (vitamin B2). It is associated with the enzymes of the class of oxido-reductases to which it is linked by a covalent bond: it is a prosthetic group.
The redox couple is: FAD ==> FADH.