Fundamentals of redox chemistry: A biological oxidation–reduction (redox) reaction always involves which of the following components or events?

Introduction / Context:
Redox reactions are the backbone of cellular energy conversion, from glycolysis and the Krebs cycle to the electron transport chain. Recognizing what must be present in any redox process is foundational for biochemistry and physiology.

Given Data / Assumptions:

• Redox implies electron transfer between chemical species.
• Oxidation is electron loss; reduction is electron gain.
• Agents are named by the process they cause in others.

Concept / Approach:
In every redox pair, one species (reducing agent) donates electrons and becomes oxidized, while the other (oxidizing agent) accepts electrons and becomes reduced. Thus, there is always a gain of electrons by one partner and a loss by the other, occurring simultaneously.

Step-by-Step Solution:
Define oxidation: loss of electrons; define reduction: gain of electrons.Identify agents: reducing agent donates; oxidizing agent accepts.Conclude: any redox reaction includes an oxidizing agent, a reducing agent, and a gain of electrons by the reduced partner.

Verification / Alternative check:
Half-reaction notation always pairs an oxidation half-reaction with a reduction half-reaction, confirming the required components.

Why Other Options Are Wrong:

• Listing any single element (A, B, or C) is incomplete; all are simultaneously true in a redox event.

Common Pitfalls:
Confusing the direction of electron flow or mislabeling agents; remember the mnemonic “OIL RIG”: Oxidation Is Loss, Reduction Is Gain.

Final Answer:
All of these