**Core Concept**
The valve of Hasner is a crucial anatomical structure located at the distal end of the nasolacrimal duct, which plays a vital role in the drainage of tears from the eye into the nasal cavity. This valve is a mucosal fold that helps regulate the flow of tears and prevents the backflow of nasal secretions into the eye.

**Why the Correct Answer is Right**
The valve of Hasner is a mucosal fold located at the junction of the nasolacrimal duct and the inferior meatus of the nasal cavity. It is composed of circular smooth muscle and mucous membrane, which contracts to prevent the backflow of nasal secretions into the eye. This valve is also known as the lacrimal papilla and is a key anatomical feature that helps maintain the health and function of the eye.

**Why Each Wrong Option is Incorrect**
**Option B:** The sphenoidal sinus opening is a separate anatomical structure located in the sphenoid bone, which is not related to the nasolacrimal duct or the valve of Hasner.
**Option C:** The frontal sinus opening is located in the frontal bone and is not associated with the nasolacrimal duct or the valve of Hasner.
**Option D:** The ethmoidal sinus opening is a collective term for the multiple openings of the ethmoidal sinuses, which are not directly related to the nasolacrimal duct or the valve of Hasner.

**Clinical Pearl / High-Yield Fact**
The valve of Hasner is an important anatomical landmark that can be relevant in clinical scenarios such as nasolacrimal duct obstruction or dacryocystitis, where the valve may become incompetent, leading to tearing or nasal discharge.

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