**Core Concept**
Ptosis refers to the drooping of the upper eyelid, which can be caused by various factors such as muscle weakness, nerve damage, or anatomical abnormalities. In some cases, ptosis can be exacerbated by specific movements or positions of the face.

**Why the Correct Answer is Right**
The type of ptosis that decreases with mouth opening is known as **Myasthenic Ptosis**. This condition is associated with myasthenia gravis, an autoimmune disease that affects the neuromuscular junction. In myasthenia gravis, the amount of acetylcholine released by nerve endings is reduced, leading to muscle weakness. When the mouth is opened, the diaphragm and other muscles of respiration are engaged, increasing the demand for acetylcholine. This increased demand can temporarily improve the transmission of nerve impulses to the extraocular muscles, resulting in a decrease in ptosis. This phenomenon is known as the **Osserman phenomenon**.

**Why Each Wrong Option is Incorrect**
**Option A:** In **Congenital Ptosis**, the amount of ptosis does not change with mouth opening, as it is a structural defect present at birth.

**Option B:** **Blepharoptosis** is a term used to describe ptosis, but it does not specifically refer to a type of ptosis that decreases with mouth opening.

**Option C:** **Horner's Syndrome** is a condition that affects the sympathetic nerves supplying the eye, leading to ptosis, but it is not directly related to the phenomenon of decreased ptosis with mouth opening.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with ptosis, it is essential to consider the possibility of myasthenic ptosis and perform tests such as the **Edrophonium test** to confirm the diagnosis.

**Correct Answer:** C.