**Core Concept**
Primary angle of deviation is the angle between the visual axis of a deviating eye and the line of sight of the other eye. This concept is crucial in understanding the types of strabismus and their management. The primary angle of deviation is different from the secondary angle of deviation, which is the angle between the visual axis of the deviating eye and the line of fixation of the other eye.

**Why the Correct Answer is Right**
In esotropia (convergent squint), the primary angle of deviation is equal to the secondary angle of deviation. This is because the deviating eye is looking inward, and the angle between the visual axis of the deviating eye and the line of sight of the other eye is the same as the angle between the visual axis of the deviating eye and the line of fixation of the other eye. This is due to the parallel alignment of the visual axes in esotropia.

**Why Each Wrong Option is Incorrect**
**Option A:** In exotropia (divergent squint), the primary angle of deviation is equal to the secondary angle of deviation, but this is not the correct squint type being referred to in the question.
**Option B:** In hypertropia (vertical strabismus), the primary and secondary angles of deviation are not equal, making this an incorrect choice.
**Option C:** In esophoria (latent squint), the primary and secondary angles of deviation are not equal, making this an incorrect choice.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between primary and secondary angles of deviation when evaluating squint, as this information helps in determining the type of strabismus and planning the appropriate treatment.

**Correct Answer: A. Esotropia.**