**Core Concept**
The question tests the understanding of **extraocular muscle function** and **ocular alignment**. Concomitant squint and paralytic squint are two types of **strabismus**, which is a disorder of eye alignment.

**Why the Correct Answer is Right**
To differentiate between concomitant and paralytic squint, a test that assesses the **extraocular muscle balance** and **ocular movement** is needed. The **cover-uncover test** and **alternate cover test** are commonly used, but the most specific test is the **Bielschowsky head tilt test**, which assesses the function of the **superior oblique muscle**.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically assess the extraocular muscle function.
**Option B:** This option is also incorrect as it is not directly related to differentiating between concomitant and paralytic squint.
**Option C:** This option is incorrect because, although it may provide some information on eye alignment, it is not the most specific test for differentiating between these two types of squint.

**Clinical Pearl / High-Yield Fact**
The **Bielschowsky head tilt test** is a crucial test in ophthalmology that helps to diagnose **superior oblique palsy**, which is a common cause of paralytic squint.

**Correct Answer:** D. Bielschowsky head tilt test.