## Core Concept
The posterior belly of the digastric muscle is a key anatomical landmark in the neck, and understanding the structures that lie deep to it is essential for surgical and clinical procedures. The digastric muscle has two bellies (anterior and posterior) connected by a tendon. The posterior belly is located in the posterior part of the neck.

## Why the Correct Answer is Right
The structures lying deep to the posterior belly of the digastric muscle include the **parotid gland**, **stylohyoid muscle**, and **retromandibular vein**. Among the given options, if one is not deep to the posterior belly of the digastric, it would be the correct answer. Typically, the **submandibular gland** is located anterior to the posterior belly of the digastric muscle and is not considered deep to it.

## Why Each Wrong Option is Incorrect
- **Option A:** This option would be incorrect because it represents a structure that is indeed deep to the posterior belly of the digastric muscle. Without the specific details of A, B, C, or D, we assume based on common anatomy that parotid gland, stylohyoid muscle, or similar structures are deep.
- **Option B:** Similarly, this would be a structure deep to the posterior belly, making it an incorrect choice as it's not the one that does not belong.
- **Option D:** This option also represents a structure that is deep, making it incorrect for the same reason.

## Clinical Pearl / High-Yield Fact
A key clinical correlation to remember is that the posterior belly of the digastric muscle serves as a landmark for identifying the **parotid gland** and the **facial nerve** during surgical procedures, as the facial nerve exits the stylomastoid foramen and runs anteriorly, superficial to the posterior belly of the digastric.

## Correct Answer: B. submandibular gland.