**Core Concept**
In acute rhinitis, the nasal mucosa's inflammatory response leads to increased mucus production. The mucus is typically clear in the initial stages of the infection, as the body's natural defense mechanism attempts to trap pathogens and debris.

**Why the Correct Answer is Right**
The correct answer is Clear because the nasal drainage in acute rhinitis is primarily composed of serous fluid and water, which gives it a clear or watery appearance. This is due to the action of the mucociliary escalator, a mechanism where the cilia on the surface of the epithelial cells lining the nasal passages beat in unison to move mucus upwards towards the nasopharynx, where it can be swallowed or expelled. In the early stages of acute rhinitis, the mucus is mostly serous and clear.

**Why Each Wrong Option is Incorrect**
**Option A:** Yellow nasal drainage is typically seen in cases of chronic rhinitis or allergic rhinitis, where the mucus becomes thickened and tinged with yellow or yellow-green due to the presence of cellular debris and inflammatory mediators.

**Option B:** Green nasal drainage is characteristic of a secondary bacterial infection, which can occur in the setting of acute rhinitis. The green coloration is due to the presence of pus, which is composed of dead white blood cells, bacteria, and other cellular debris.

**Option D:** Gray nasal drainage is not a typical description of nasal discharge in acute rhinitis. Gray or discolored nasal discharge may be seen in cases of sinusitis or other chronic conditions, but it is not a common finding in acute rhinitis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that changes in nasal drainage color can indicate the progression of the infection or the development of complications. A yellow or green tint may suggest a secondary bacterial infection, while a sudden increase in nasal discharge or a change in its consistency may indicate a sinusitis or other complications.

**✓ Correct Answer: C. Clear.**