**Core Concept**
Atherosclerosis is a chronic inflammatory disease characterized by the buildup of lipid-rich plaques within the arterial walls, leading to arterial narrowing and increased risk of cardiovascular events. The assessment of plaque characteristics is crucial for determining the risk of rupture and subsequent thrombosis.

**Why the Correct Answer is Right**
Intravascular ultrasound (IVUS) is the best investigation for assessing the plaque of atherosclerosis because it provides high-resolution images of the arterial wall and plaque morphology. IVUS can accurately measure plaque size, lipid content, and calcification, which are critical factors in determining plaque vulnerability. Additionally, IVUS can guide interventional procedures, such as plaque stabilization and stenting.

**Why Each Wrong Option is Incorrect**
**Option A:** Angiography is a useful tool for visualizing the luminal narrowing caused by atherosclerosis, but it does not provide detailed information about plaque composition or morphology.
**Option B:** Computed tomography angiography (CTA) can assess plaque calcification and burden, but it is limited by radiation exposure and may not accurately detect non-calcified plaques.
**Option C:** Magnetic resonance imaging (MRI) can provide information about plaque composition and inflammation, but it is not as widely available or cost-effective as IVUS.

**Clinical Pearl / High-Yield Fact**
IVUS can help identify high-risk plaques, which are characterized by a large lipid core, thin fibrous cap, and high levels of inflammation. This information can guide therapeutic decisions, such as the use of statins, anti-inflammatory agents, or plaque stabilization procedures.

**Correct Answer:** C. Intravascular ultrasound (IVUS)