**Core Concept**
The resorption of the primary tooth root is a natural process that occurs as part of the normal development and replacement of primary teeth with permanent teeth. This process is initiated by the activation of osteoclasts, which are cells responsible for bone resorption.

**Why the Correct Answer is Right**
The resorption of the primary tooth root typically begins around 3 years of age, when the permanent successor tooth starts to erupt into the oral cavity. This process is triggered by the mechanical pressure exerted by the growing permanent tooth, which stimulates the surrounding bone to resorb the primary root. The resorption process is mediated by the activation of osteoclasts, which express specific receptors for RANKL (receptor activator of NF-κB ligand) and are responsive to the mechanical pressure exerted by the growing permanent tooth.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 year approx - This is too early for the resorption of the primary tooth root to begin, as the permanent successor tooth has not yet started to erupt.
**Option B:** 2 year approx - While some resorption may start at this age, it is generally not considered a reliable marker for the initiation of root resorption.
**Option D:** 4 year approx - This is too late, as the resorption process typically begins around 3 years of age.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the timing of primary tooth root resorption can vary depending on various factors, including the type of primary tooth, the position of the permanent successor tooth, and the overall oral health of the individual.

**✓ Correct Answer: C. 3 year approx**