**Core Concept**
Branchial cysts, also known as branchial cleft cysts, are congenital anomalies resulting from the incomplete closure of branchial clefts during embryonic development. These cysts are typically lined by keratinizing stratified squamous epithelium and can present in various locations along the anterior border of the sternocleidomastoid muscle.

**Why the Correct Answer is Right**
The correct answer, **A. It occurs at lower 2/3rd of sternocleidomastoid**, is incorrect because branchial cysts usually occur in the upper third of the anterior border of the sternocleidomastoid muscle, rather than the lower 2/3rd. This is due to the normal embryological development of the branchial clefts, which typically closes before reaching the lower third of the sternocleidomastoid muscle.

**Why Each Wrong Option is Incorrect**
**Option B:** This statement is true, as branchial cysts are indeed lined by squamous epithelium, which is a characteristic feature of these congenital anomalies.
**Option C:** This statement is also true, as branchial cysts most commonly develop from remnants of the second branchial cleft, although they can occasionally arise from the first or third branchial clefts.
**Option D:** This statement is true, as branchial cysts typically present in early or middle adulthood, often as a painless neck mass.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of branchial cysts is their characteristic location along the anterior border of the sternocleidomastoid muscle, often in the upper third. This knowledge can aid in differentiating branchial cysts from other congenital or acquired neck masses.

**✓ Correct Answer: A. It occurs at lower 2/3rd of sternocleidomastoid**