**Core Concept:**
The question is testing the knowledge of the spinal nerves and their functions. In this context, the question is asking about a specific spinal nerve that is injured, leading to the inability to look downward and laterally. The spinal nerves are classified based on their dorsal (posterior) or ventral (anterior) root, as well as their level in the spinal cord.

**Why the Correct Answer is Right:**
The correct answer, D, refers to the C5 spinal nerve. The C5 nerve is a ventral (anterior) spinal nerve that arises from the cervical spinal cord, specifically from the second cervical (C2-C6) spinal cord segment. This nerve has several functions, including:

1. Controlling the deltoid muscle, which is responsible for shoulder elevation and external rotation. Inability to look downward (infraspinatus muscle) and laterally (scapular stabilization) is a result of the impaired function of the C5 nerve.
2. Contributing to the sensation of the anterior chest wall and the lateral aspects of the arm. This is relevant because the injury to this nerve would cause loss of sensation in those areas.

**Why Each Wrong Option is Incorrect:**

A. The correct answer (C5) is not one of the options provided.
B. This option refers to the C6 spinal nerve, which is incorrect because it controls the biceps muscle and does not affect shoulder elevation or external rotation.
C. This option refers to the C7 spinal nerve, which is incorrect because it controls the triceps muscle and does not affect shoulder elevation or external rotation.

**Clinical Pearl:**

In clinical practice, identifying the level of spinal nerve injury can be crucial for appropriate treatment planning and prognosis. For instance, knowing the level of injury allows for targeted physical therapy and rehabilitation exercises, as well as informing decisions about surgical intervention if needed. Additionally, understanding the functions of specific spinal nerves helps in assessing the extent of the injury and its potential long-term implications for the patient.