**Core Concept**
The corpus callosum is a thick bundle of nerve fibers connecting the two hemispheres of the brain, enabling communication between them. It has distinct regions, each with unique characteristics.

**Why the Correct Answer is Right**
The splenium is the posterior (rear) part of the corpus callosum, connecting the occipital lobes of the two hemispheres. This region is crucial for visual processing and integration between the two hemispheres. The splenium is also the thickest part of the corpus callosum and contains the largest number of nerve fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** The rostrum is the anterior (front) part of the corpus callosum, connecting the frontal lobes of the two hemispheres. It does not match the description given in the question.
**Option B:** The genu is the anterior (front) part of the corpus callosum, connecting the frontal lobes of the two hemispheres. It is not the posterior part described in the question.
**Option C:** The body is the middle part of the corpus callosum, connecting the parietal and temporal lobes of the two hemispheres. It is not the posterior part described in the question.

**Clinical Pearl / High-Yield Fact**
Damage to the corpus callosum, particularly the splenium, can lead to difficulties in integrating visual information between the two hemispheres, resulting in impaired visual processing and coordination.

**✓ Correct Answer: D. Splenium**