**Core Concept**
The question tests knowledge of anatomical relationships between the appendix and surrounding muscles, specifically in the context of a retrocaecal appendix. A retrocaecal appendix is located behind the caecum, near the **psoas major muscle**.

**Why the Correct Answer is Right**
In the case of a retrocaecal appendix, the **psoas major muscle** is the one that can become irritated due to its proximity to the inflamed appendix. This irritation can lead to **psoas irritation sign**, a clinical sign where patients experience pain upon extension of the hip.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not directly related to the typical location of a retrocaecal appendix.
**Option B:** Incorrect as it is not the primary muscle irritated in this scenario.
**Option C:** Incorrect because, although involved in abdominal movements, it's not the specific muscle irritated by a retrocaecal appendix.

**Clinical Pearl / High-Yield Fact**
Remember, a retrocaecal appendix can cause **psoas irritation**, leading to hip pain upon extension, which is a critical clinical correlation for diagnosing appendicitis in such cases.

**Correct Answer:** D. Psoas major muscle