Step 1: Define a ranula. A ranula is a cystic swelling in the floor of the mouth arising from the sublingual or submandibular salivary gland, named for its resemblance to a frog belly (rana = frog).
Step 2: Assess option B. It is a cystic swelling in the floor of the mouth, which is the defining and most directly true description of a ranula. This is the intended answer.
Step 3: Clarify option D. A ranula is indeed a mucus extravasation or retention type cyst, so option D is also conceptually true, but option B is the single best and most specific defining statement asked for here.
Step 4: Eliminate the false options. Epulis is a gum (gingival) swelling, not a ranula, so option A is wrong. A thyroglossal cyst is a midline neck swelling along the thyroglossal tract, unrelated to the floor of mouth, so option C is wrong.
Step 5: Conclusion. The best answer is that a ranula is a cystic swelling in the floor of the mouth, option 2.