**Core Concept**
The sorting of proteins to their final destinations within the cell is a crucial process that determines their function and fate. In the context of lysosomal targeting, a specific signal is required to guide proteins to the lysosomes, where they can participate in various cellular processes such as degradation and recycling.

**Why the Correct Answer is Right**
The signal that directs a protein to the lysosomes is the lysosomal targeting signal, also known as Lysosomal Targeting Signal (LTS). This signal is typically a short peptide sequence that is recognized by the mammalian target of rapamycin (mTOR) complex, which then interacts with the Golgi apparatus to ensure the protein is transported to the lysosomes. The LTS is often embedded within the protein sequence and is essential for its proper localization and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a known signal for lysosomal targeting. While some proteins may be targeted to lysosomes through other mechanisms, the LTS is a well-characterized signal for this process.
**Option B:** This option is incorrect because the KDEL sequence is a retrieval signal that targets proteins back to the endoplasmic reticulum (ER), not to the lysosomes.

**Clinical Pearl / High-Yield Fact**
The lysosomal targeting signal is a crucial determinant of protein function and fate, and its disruption or mutation can lead to various lysosomal storage diseases. Understanding the mechanisms of lysosomal targeting is essential for the diagnosis and treatment of these disorders.

**Correct Answer:** C.