**Core Concept**
Under normal resting conditions, the partial pressure of oxygen (PO2) in arterial blood is maintained within a narrow range to ensure optimal oxygen delivery to tissues. Haemoglobin oxygen saturation (SaO2) is a critical parameter that reflects the balance between oxygen delivery and consumption.

**Why the Correct Answer is Right**
A normal resting condition is characterized by adequate oxygen delivery to tissues, which is reflected in a high haemoglobin oxygen saturation in systemic arterial blood. The normal range of SaO2 is typically above 95%, with an average value of approximately 97%. This range ensures that there is sufficient oxygen available to meet the metabolic demands of tissues. The partial pressure of oxygen in arterial blood (PaO2) is also maintained within a narrow range, typically between 75 and 100 mmHg, which corresponds to a high SaO2. This balance is crucial for maintaining normal cellular function and preventing tissue hypoxia.

**Why Each Wrong Option is Incorrect**
**Option A:** A range of 25 to 75% would be consistent with severe tissue hypoxia and would not represent a normal resting condition.
**Option B:** A range of 40 to 75% would indicate significant tissue hypoxia and would not be compatible with normal cellular function.
**Option C:** A range of 40 to 95% would be consistent with mild tissue hypoxia and would not represent a normal resting condition.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a SaO2 below 90% can indicate tissue hypoxia, and values below 80% can lead to significant cellular dysfunction.

**✓ Correct Answer: D. 75 to 98%**