**Core Concept**
The investing layer of deep cervical fascia is a part of the deep cervical fascia that envelops the neck, providing support to the underlying structures. This fascial layer is continuous with the superficial fascia of the chest and the anterior abdominal wall, forming a continuous sheath around the neck and its contents.

**Why the Correct Answer is Right**
The investing layer of deep cervical fascia encloses the sternocleidomastoid and trapezius muscles, as well as the two salivary glands (parotid and submandibular). However, it does not enclose the axillary vessels, which are located in the axilla and are not part of the neck's fascial envelope. The axillary vessels, including the axillary artery and vein, are enclosed by the axillary fascia, which is a separate fascial layer.

**Why Each Wrong Option is Incorrect**
**Option A:** Sternocleidomastoid and trapezius muscles are indeed enclosed by the investing layer of deep cervical fascia, which provides support to these muscles.
**Option B:** The two salivary glands (parotid and submandibular) are also enclosed by the investing layer of deep cervical fascia, as part of the parotid and submandibular gland's fascial envelopes.
**Option D:** There are two spaces enclosed by the investing layer of deep cervical fascia, including the carotid sheath and the prevertebral fascial space.

**Clinical Pearl / High-Yield Fact**
The investing layer of deep cervical fascia is a key anatomical structure that provides support to the neck's contents, including muscles, glands, and vessels. Understanding the fascial relationships in the neck is crucial for surgical procedures, such as neck dissections, and for diagnosing conditions that affect the neck's fascial structures.

**✓ Correct Answer: C. Axillary vessels**