**Core Concept**
The fecal-oral route of transmission is a common mode of spread for various infectious agents, typically involving the ingestion of fecally contaminated food or water. This route is often associated with enteric pathogens that can cause gastrointestinal infections.

**Why the Correct Answer is Right**
The correct answer is hepatitis A virus (HAV). HAV is primarily transmitted through the fecal-oral route, where contaminated food or water is ingested, allowing the virus to enter the host's gastrointestinal tract. This mode of transmission is facilitated by the virus's ability to remain viable in the environment and on contaminated surfaces. The liver is the primary target organ for HAV, leading to symptoms such as jaundice, fatigue, and abdominal pain.

**Why Each Wrong Option is Incorrect**
**Option A:** Norovirus is also transmitted through the fecal-oral route but is more commonly associated with outbreaks of gastroenteritis in closed environments, such as cruise ships and healthcare facilities.
**Option B:** Rotavirus is primarily transmitted through the fecal-oral route and is a major cause of severe diarrhea in children worldwide, but it is more commonly associated with severe gastroenteritis in infants and young children.
**Option C:** Hepatitis B virus (HBV) is primarily transmitted through parenteral routes, such as needlestick injuries or blood transfusions, rather than the fecal-oral route.

**Clinical Pearl / High-Yield Fact**
HAV is highly contagious and can spread rapidly in areas with poor sanitation and hygiene, making vaccination an essential tool in preventing outbreaks.

**Correct Answer: C. Hepatitis A virus. Hepatitis A virus.**