The correct option is B Mobile e-carrier of membrane
The metabolic pathway through which electron passes from one carrier to another is called electron transport system or chain and also known as mitochondrial respiratory chain. The ETS in mitochondria consist of four complexes, found in bases of stalked particles in the inner mitochondrial membrane, ubiquinone or co-enzyme Q and cytochrome c (not bound to stalked particles but acts as mobile electron carrier between the complexes. Complex-lll consist of ubiquinol, (reduced ubiquinone), cytochrome c and cytochrome b,c. Cytochrome c is a small protein attached to the outer surface of inner mitochondrial membrane and acts as a mobile carrier for transfer of electrons between complex-lll and IV, which is called as $QH_2$−QH2− cytochrome c reductase complex. While complex-IV is known as cytochrome c oxidase complex.
Hence, the correct answer is mobile e-carrier of membrane'