**Core Concept**
The clinical presentation of this diarrheal illness, characterized by voluminous, watery stools, accompanied by headaches, abdominal cramping pain, and myalgias, is suggestive of a viral gastroenteritis. The illness is self-limiting, lasting only 1 to 3 days and resolving without sequelae.

**Why the Correct Answer is Right**
The most likely cause of this illness is Norovirus, a highly contagious RNA virus that primarily affects the gastrointestinal tract. Norovirus causes a range of symptoms, including diarrhea, abdominal cramping, nausea, and vomiting, which are consistent with the presentation described. The virus is known to spread rapidly in closed environments, such as resorts, and can cause outbreaks among large groups of people. The laboratory findings of no increase in leukocytes or fat and no RBCs in the stool samples are consistent with a viral etiology.

**Why Each Wrong Option is Incorrect**
**Option A:** Rotavirus is a common cause of severe diarrhea in children, but it is less likely to cause outbreaks in adults and typically presents with more severe symptoms.
**Option B:** Salmonella is a bacterium that can cause diarrhea, but it typically presents with fever, abdominal pain, and bloody stools, which are not consistent with the described case.
**Option C:** Clostridioides difficile (C. diff) is a bacterium that causes diarrhea, but it is typically associated with antibiotic use and hospital-acquired infections, which is not consistent with the resort setting.

**Clinical Pearl / High-Yield Fact**
Norovirus is highly contagious and can be spread through contaminated food and water, as well as through person-to-person contact. It's essential to practice good hand hygiene and proper food handling to prevent outbreaks of Norovirus gastroenteritis.

**Correct Answer:** C. Norovirus.