Ans. is 'b' i.e., Inhibitor of oxidative phosphorylationInhibitors of electron transpo chain?Inhibitors of respiratory chain may be divided into three groups : ?1. Inhibitors of electron transpo chain properThese inhibitors inhibit the flow of electrons through the respiratory chain. This occurs at following sites.Complex I (NADH to CoQ) is inhibited by : - Barbiturates (amobarbital), Piericidin A (an antibiotic), rotenone (an insectiside), chlorpromazine (a tranquilizer), and guanethidine (an ntihypeensive). These inhibitors block the transfer of reducing equivalents from FeS protein to CoQ.Complex II is inhibited by : - Carboxin and TTFA inhibit transfer of electon from FADH2 to CoQ, whereas malanate competitively inhibit from succinate to complex II. Complex III (Cytochrome b to cytochrome CI) is inhibited by : - Dimercaprol, antimycin A, BAL(British antilewisite), Naphthyloquinone. These inhibitors block the transfer of electrons from cytochrome b to cytochromeComplex IV (cytochrome C oxidase) is inhibited by : - Carbon monoxide, CN-, H2S and azide (N3-). These inhibitors block the transfer of electrons from cytochrome aa3 to molecular oxygen and therefore can totally arrest cellular respiration.2. Inhibitors of oxidative phosphorylationThese compounds directly inhibit phosphorylation of ADP to ATP. Oligomycin inhibits Fo component of F0F, ATPase. Atractiloside inhibits translocase, a transpo protein that transpos ADP into mitochondria for phosphorylation into ATP.3. UncouplesAs the name suggests, these componds block the coupeling of oxidation with phosphorylation. These compounds allow the transfer of reducing equivalents in respiratory chain but prevent the phosphorylation of ADP to ATP by uncoupling the linkage between ETC and phosphorylation. Thus the energy instead of being trapped by phosphorylation is dissipated as heat. Uncouplers may be :-Natural :- Thermogenin, thyroxineSynthetic :- 2, 4-dinitrophenol (2, 4-DNP), 2, 4-dinitrocresol (2, 4-DNC), and CCCP (chlorocarbonylcyanidephenyl hydrazone).