Respiratory substrates are the organic substances which are _______during respiration to liberate energy.

Options

A. oxidised
B. reduced
C. synthesised
D. both (a) and (b)

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about respiratory substrates and what happens to them during respiration to release energy.

2. **Defining Respiratory Substrates**: Respiratory substrates are organic substances that serve as fuel for respiration. They are primarily macromolecules like carbohydrates, fats, and proteins.

3. **Identifying the Process of Respiration**: Respiration is an oxidative process. This means that during respiration, these substrates undergo oxidation.

4. **Oxidation and Energy Liberation**: When respiratory substrates are oxidized, they release energy. This energy is essential for various cellular processes.

5. **Conclusion**: Based on the understanding that respiratory substrates are oxidized during respiration to liberate energy, the correct answer to fill in the blank is "oxidized."

**Final Answer**: Respiratory substrates are the organic substances which are **oxidized** during respiration to liberate energy.

---