**Core Concept**
The colorimetric test, also known as the Schirmer's test, is a method used to assess the function of the lacrimal gland by measuring the amount of tears produced in response to stimulation. This test evaluates the basal tear production and the reflex tear production in response to irritation of the conjunctiva.

**Why the Correct Answer is Right**
The Schirmer's test involves placing a small strip of filter paper under the eyelid to measure the amount of tears produced over a set period of time. The normal response to the test is a wetting of the strip by the tears, indicating adequate lacrimal gland function. The test is useful in diagnosing conditions such as dry eye syndrome, where the lacrimal gland is not producing enough tears to maintain the health of the cornea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the colorimetric test is not used to assess the function of the salivary gland. The salivary gland is responsible for producing saliva, which is different from tears.

**Option B:** This option is incorrect because the colorimetric test is not used to assess the function of the parotid gland. The parotid gland is a type of salivary gland that produces saliva.

**Option C:** This option is incorrect because the colorimetric test is not used to assess the function of the tear duct. The tear duct is a small tube that drains tears from the eye into the nasal cavity.

**Clinical Pearl / High-Yield Fact**
The Schirmer's test is a useful diagnostic tool in ophthalmology to assess the function of the lacrimal gland and diagnose conditions such as dry eye syndrome.

**Correct Answer: D. Lacrimal gland.**