**Core Concept**
Association fibers are a type of white matter tract in the brain that connect different regions within the same hemisphere, enabling the integration and processing of various sensory and motor information.

**Why the Correct Answer is Right**
Association fibers play a crucial role in facilitating communication between different cortical areas, allowing for the coordination of complex behaviors and the processing of sensory information. These fibers are primarily composed of myelinated axons that transmit signals between cortical regions, such as the primary sensory cortex, secondary sensory cortex, and motor cortex. The two main types of association fibers are the callosal fibers (which connect the two hemispheres) and the intrahemispheric fibers (which connect different regions within the same hemisphere).

**Why Each Wrong Option is Incorrect**
**Option A:** Corticospinal tracts - These are motor tracts that carry signals from the cortex to the spinal cord, not association fibers.
**Option B:** Sensory tracts - Sensory tracts, such as the dorsal column-medial lemniscus pathway, carry sensory information from the periphery to the brain, but are not association fibers.
**Option C:** Commissural fibers - Commissural fibers, such as the corpus callosum, connect the two hemispheres, but are not intrahemispheric association fibers.

**Clinical Pearl / High-Yield Fact**
Association fibers are responsible for the integration of sensory information from different modalities, such as vision, hearing, and touch, allowing for the perception of a unified sensory experience.

**Correct Answer: D. Different regions within the same hemisphere.**