**Core Concept**
A ranula is a type of mucocele, which is a mucus-containing cyst, typically arising from the sublingual gland. This condition is characterized by the accumulation of mucus within the gland, leading to the formation of a cystic structure. The sublingual gland is a minor salivary gland located under the tongue, and its dysfunction can result in the development of a ranula.

**Why the Correct Answer is Right**
The correct answer, **Option A: Retention cyst of sublingual gland**, is accurate because a ranula is indeed a type of retention cyst that arises from the sublingual gland. A retention cyst occurs when the glandular ducts become obstructed, leading to the accumulation of mucus within the gland. This obstruction can be due to various factors, including trauma, infection, or inflammation. The sublingual gland is particularly prone to developing retention cysts, which can manifest as a ranula.

**Why Each Wrong Option is Incorrect**
**Option B:** A retention cyst of the submandibular gland is known as a **sialocele** or **parotid cyst**, not a ranula. The submandibular gland is a major salivary gland located under the jaw, and its dysfunction can result in the development of a sialocele or parotid cyst.

**Option C:** An extravasation cyst of the sublingual glands is not a recognized medical term. A mucocele, including a ranula, is a type of retention cyst, not an extravasation cyst.

**Option D:** An extravasation cyst of the submandibular glands is not a recognized medical term. A sialocele or parotid cyst is a type of retention cyst, not an extravasation cyst.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate a ranula from other types of oral lesions, such as a mucocele or a sialocele. A ranula typically presents as a blue-colored, painless swelling under the tongue, and its diagnosis can be confirmed with imaging studies or histopathological examination.

**✓ Correct Answer: A. Retention cyst of sublingual gland**