**Core Concept**
Hep-2 cells are a type of cell line used in research and diagnostic applications, particularly for the isolation and cultivation of viruses. They are derived from human laryngeal carcinoma cells and are characterized by their ability to be cultured indefinitely in the laboratory.

**Why the Correct Answer is Right**
Hep-2 cells are a continuous cell line, meaning they can be cultured for extended periods of time without significant changes in their growth characteristics or morphology. This is in contrast to primary cell cultures, which are derived from freshly harvested tissues and have a limited lifespan. The continuous nature of Hep-2 cells makes them useful for applications such as virus isolation and diagnostic testing. They are also immortalized, meaning they can divide indefinitely, which is a characteristic of cancer cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Primary cell cultures are derived from freshly harvested tissues and have a limited lifespan, whereas Hep-2 cells are a continuous cell line that can be cultured indefinitely.
* **Option B:** Diploid cell strains are typically derived from normal cells and have a finite lifespan, whereas Hep-2 cells are aneuploid and immortalized.
* **Option D:** Explant culture involves the growth of cells in a three-dimensional matrix, whereas Hep-2 cells are grown in a two-dimensional culture system.

**Clinical Pearl / High-Yield Fact**
Hep-2 cells are commonly used for the diagnosis of respiratory viruses, including influenza and adenovirus, and are also useful for the isolation of other viruses such as herpes simplex and human papillomavirus.

**✓ Correct Answer: C. Continuous cell lines**