**Core Concept**
The Eustachian tube plays a crucial role in maintaining middle ear pressure and preventing otitis media by regulating airflow and pressure. Its patency is essential for proper middle ear function.

**Why the Correct Answer is Right**
The Eustachian tube patency is primarily assessed by the Toynbee maneuver, which involves swallowing while the mouth and nose are closed. This action helps to equalize the middle ear pressure and assess the function of the Eustachian tube. The Toynbee maneuver stimulates the tensor veli palatini muscle, which contracts and opens the Eustachian tube, allowing air to enter the middle ear and equalize the pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Toynbee maneuver is the primary method for assessing Eustachian tube patency, not the Valsalva maneuver. The Valsalva maneuver primarily increases intrathoracic pressure and is not used for assessing middle ear pressure.

**Option B:** This option is incorrect because the Eustachian tube is not assessed by the nasal decongestant spray. While nasal decongestant sprays may help relieve Eustachian tube obstruction by reducing nasal congestion, they do not directly assess patency.

**Option C:** This option is incorrect because the tympanometry is a test used to assess the function of the middle ear system, including the Eustachian tube, but it is not the primary method for assessing Eustachian tube patency. Tympanometry can provide information about the middle ear pressure and compliance, but it is not as direct as the Toynbee maneuver.

**Option D:** This option is incorrect because the acoustic reflex is a reflex that protects the ear from loud sounds, but it is not directly related to assessing Eustachian tube patency.

**Clinical Pearl / High-Yield Fact**
The Toynbee maneuver is a simple and effective way to assess Eustachian tube patency and can be used in various clinical settings to evaluate middle ear function.

**Correct Answer: . Toynbee maneuver**