The redox carriers of the electron transport chain that have a tightly bound metal atom responsible for accepting and donating electrons are

a. nad
b. fad
c. nadp
d. the cytochromes
e. the flavoproteins

The correct answer is D. The cytochromes

Explanation:

Cytochromes are proteins which contain tightly bound heme metal which easily undergoes oxidation and reduction and are responsible for accepting and donating the electron. They play an important role in the electron transport chain and helps in formation of ATP during oxidative phosphorylation.

The metal ion provides the capability to the cytochrome to gain and loss the electron. a, b, and c are the three classes of cytochrome out of which cytochrome c is the most abundant and stable.  

Therefore the redox carrier of ETC that have bounded metal atom are cytochromes.