**Core Concept**
Ranula is a type of mucous retention cyst that occurs in the floor of the mouth, specifically in the sublingual gland or under the sublingual frenulum. It is caused by the obstruction of the sublingual gland's duct, leading to the accumulation of mucus.

**Why the Correct Answer is Right**
The correct answer is related to the pathogenesis of ranula. Ranula forms due to the obstruction of the sublingual gland's duct, which can be caused by trauma, infection, or other factors leading to inflammation and swelling. This obstruction prevents the normal drainage of mucus from the gland, resulting in the formation of a mucous retention cyst. The sublingual gland's duct is a small structure that connects the gland to the oral cavity, and its obstruction can lead to the accumulation of mucus, which can eventually form a ranula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ranula is not a type of tumor. While tumors can occur in the floor of the mouth, ranula is a benign cystic structure that forms due to the obstruction of the sublingual gland's duct.
**Option B:** This option is incorrect because ranula is not a type of abscess. While abscesses can form in the floor of the mouth, ranula is a specific type of mucous retention cyst that forms due to the obstruction of the sublingual gland's duct.
**Option C:** This option is incorrect because ranula is not a type of cyst that occurs in the tongue. While cysts can occur in the tongue, ranula is a specific type of cyst that occurs in the floor of the mouth.

**Clinical Pearl / High-Yield Fact**
A ranula can be differentiated from a mucocele by its location and size. A ranula is typically located on the floor of the mouth, while a mucocele is typically located on the lip or tongue. Additionally, a ranula is often larger than a mucocele.

**Correct Answer: D. Ranula is a type of mucous retention cyst that occurs in the floor of the mouth.**