**Core Concept**
Hasner's valve, also known as the valve of Hasner, is a mucosal fold located at the distal end of the nasolacrimal duct. It is a crucial anatomical structure that regulates the flow of tears from the eye into the nasal cavity.

**Why the Correct Answer is Right**
The valve of Hasner is situated at the junction of the nasolacrimal duct and the inferior meatus of the nasal cavity. It is a mucosal fold that prevents the backflow of nasal mucus into the nasolacrimal duct, thereby maintaining the integrity of the tear drainage system. The valve of Hasner is composed of mucous membrane and is located approximately 1 cm from the nasal orifice of the nasolacrimal duct.

**Why Each Wrong Option is Incorrect**
**Option B:** The sphenoidal sinus opening is located in the posterior aspect of the nasal cavity, near the sphenoid bone. It is not related to the nasolacrimal duct or the valve of Hasner.
**Option C:** The frontal sinus opening is located in the anterior aspect of the nasal cavity, near the frontal bone. It is not related to the nasolacrimal duct or the valve of Hasner.
**Option D:** The ethmoidal sinus opening is located in the middle meatus of the nasal cavity, near the ethmoid bone. It is not related to the nasolacrimal duct or the valve of Hasner.

**Clinical Pearl / High-Yield Fact**
The valve of Hasner is an important anatomical landmark in ophthalmology and otolaryngology. Knowledge of its location and function is essential for understanding the pathophysiology of nasolacrimal duct obstruction and for performing procedures such as nasolacrimal duct irrigation.

**✓ Correct Answer: A. Opening of nasolacrimal duct**