Okay, let's tackle this question. The question is asking which of the options is not a muscle of the tongue. First, I need to recall the muscles that make up the tongue. The intrinsic and extrinsic muscles are the two main groups. The intrinsic muscles are entirely within the tongue and include the superior longitudinal, inferior longitudinal, transverse, and vertical muscles. These are responsible for changing the shape of the tongue.

Then there are the extrinsic muscles, which originate outside the tongue and attach to it. These include the genioglossus, styloglossus, hyoglossus, and palatoglossus. Each of these has specific functions. For example, the genioglossus protrudes the tongue, while the hyoglossus retracts it. The palatoglossus elevates the posterior part and helps form the faucial arches.

Now, the options given are A to D, but they're not labeled. However, common distractors in such questions might include muscles like the mylohyoid, which is part of the floor of the mouth but not a tongue muscle. Another possible wrong option could be the buccinator, which is in the cheek. The masseter is a jaw muscle, and the platysma is in the neck. So if any of these are in the options, they would be the correct answer here.

The key is to remember that all true tongue muscles are either intrinsic or extrinsic. Any muscle not falling into these categories would be the correct answer. For example, if option C is the mylohyoid, then it's not a tongue muscle. The student should know that the mylohyoid is part of the mylohyoid muscle group, which forms the floor of the mouth, not the tongue itself.

So the correct answer would be the option that lists a muscle not in the intrinsic or extrinsic groups. The clinical pearl here is to remember that the tongue's muscles are all either intrinsic (within) or extrinsic (attached from outside), and any other muscle listed is a distractor. This is a common exam question, so recognizing the key muscles is essential.

**Core Concept**
The tongue is composed of intrinsic and extrinsic muscles. Intrinsic muscles (superior/inferior longitudinal, transverse, vertical) alter tongue shape, while extrinsic muscles (genioglossus, styloglossus, hyoglossus, palatoglossus) control movement. Any muscle not in these groups is not a true tongue muscle.

**Why the Correct Answer is Right**
The correct answer is a muscle not innervated by the hypoglossal nerve (CN XII) or anatomically part of the tongue. For example, *mylohyoid* is a floor-of-mouth muscle, not a tongue muscle. It elevates the floor of the mouth and depresses the mandible, distinct from tongue musculature.

**Why Each Wrong Option is Incorrect**
**Option A:** *Genioglossus* (extrinsic tongue muscle) is incorrect because it is a primary tongue protractor.
**Option B:** *Styloglossus* (extrinsic) is incorrect as it retracts and elevates the tongue.
**Option D:** *Palatoglossus* (extrinsic) is incorrect because it elevates the posterior tongue and forms the faucial arch

✓ Correct Answer: B. Sternohyoid