In electron transport system, the final acceptor of electrons is :

Options

A. oxygen
B. cytochrome `a_3`
C. cytochrome b
D. `NADH_2`

Explanation

### Step-by-Step Solution:

1. **Understanding the Electron Transport System (ETS)**: The electron transport system is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions. This process is crucial for cellular respiration and occurs in the inner mitochondrial membrane in eukaryotes.

2. **Identifying the Source of Electrons**: The electrons used in the electron transport chain are primarily derived from the breakdown of glucose during cellular respiration. In photosynthesis, electrons are generated from the photolysis of water, which splits water molecules into hydrogen ions and electrons.

3. **Role of Electron Carriers**: The electrons are transported through various electron carriers, which include cytochromes (like cytochrome b and cytochrome a3) and other proteins. These carriers facilitate the transfer of electrons through the electron transport chain.

4. **Final Electron Acceptor**: As the electrons move through the chain, they eventually reach a final electron acceptor. In aerobic respiration, this final acceptor is molecular oxygen (O2).

5. **Formation of Water**: When the electrons combine with molecular oxygen and hydrogen ions (H+), they form water (H2O). This reaction is crucial as it helps maintain the flow of electrons through the electron transport chain and is a key step in the process of oxidative phosphorylation.

6. **Conclusion**: Therefore, the final acceptor of electrons in the electron transport system is oxygen.

### Final Answer:
The final acceptor of electrons in the electron transport system is **oxygen**.

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