**Core Concept**
Tau proteins are a group of microtubule-associated proteins (MAPs) that play a crucial role in maintaining the structure and stability of microtubules in neurons. Abnormal tau protein aggregation is a hallmark of neurodegenerative diseases such as Alzheimer's disease.

**Why the Correct Answer is Right**
The correct answer is associated with Alzheimer's disease, a neurodegenerative disorder characterized by the accumulation of tau protein aggregates in the form of neurofibrillary tangles. This process disrupts normal microtubule function, leading to neuronal dysfunction and death. The tau protein aggregates are thought to be a downstream consequence of amyloid-beta peptide accumulation, which triggers a cascade of pathological events in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because tau proteins are not primarily associated with prion diseases, which are caused by misfolded prion proteins. While tau protein aggregation can occur in prion diseases, it is not the most common association.
**Option B:** This option is incorrect because tau proteins are not primarily associated with frontotemporal dementia (FTD), although tau protein aggregation can be a feature of some FTD subtypes. The primary association of tau proteins is with Alzheimer's disease.
**Option C:** This option is incorrect because tau proteins are not primarily associated with Huntington's disease, which is caused by an expansion of a CAG repeat in the Huntingtin gene. While tau protein aggregation can occur in Huntington's disease, it is not the most common association.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that tau protein aggregation is a hallmark of neurodegenerative diseases, and its presence can be used as a diagnostic marker in the context of Alzheimer's disease.

**Correct Answer:** D.