In an electron transport chain, in terminal oxidation, the cytochrome which donates electrons to $O_2$ is

The correct option is D Cytochrome $a_3$
An electron transport chain is a series of compounds that transfer electrons from electron donors to electron acceptors via redox reactions and couples this electron transfer with the transfer of protons across a membrane. This creates an electrochemical proton gradient that drives ATP synthesis. Cytochrome $a_3$ is terminal cytochrome of ETC. It has $F{e}^{+++}$ and $C{u}^{++}$ both. With $F{e}^{+++}$, it picks up electrons and through $C{u}^{++}$ it passes electrons to oxygen.