Flavin proteins capable of reducing ubiquinone.
a. citric acid cycle
b. cytochrome
c. fad
d. fmn (part of complex 1)
e. glyoxylate cycle
f. quinol
g. quinone
h. nadh

The correct answer are options c and d.

Explanation:

In mitochondrial ETC for oxidative phosphorylation, two electrons of NADH dehydrogenase or complex I mediate through the FMN, FeS, and ubiquinone centers. When the electrons are transmitted to ubiquinone, it takes up two protons from the matrix to produce UQH2 or ubiquinol.  

A pool of UQH2 is produced within the membrane. The complex II produces one UQH2 per oxidation of succinate. It attains its electrons from the oxidation of succinate solely and provides them through FAD and FeS clusters to the pool of UQH2. With the assistance of FAD, FMN, FeS partial reduction of ubiquinone produces UQH, which is fatal and must be reduced to UQH2 briskly.  

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products