**Core Concept**
The question tests the student's understanding of the characteristic pupil changes in acute congestive glaucoma, also known as acute angle-closure glaucoma. This condition is characterized by a sudden increase in intraocular pressure (IOP) due to blockage of the trabecular meshwork, leading to corneal edema, pain, and vision loss.

**Why the Correct Answer is Right**
In acute congestive glaucoma, the pupil becomes oval and vertical due to corneal edema and increased IOP. The cornea swells, causing the pupil to become more elliptical and vertically oriented. This is because the cornea's curvature is altered, leading to a change in the shape of the pupil. The increased IOP also causes the iris to bow forward, further contributing to the oval and vertical shape of the pupil.

**Why Each Wrong Option is Incorrect**
**Option A:** An oval and horizontal pupil is not characteristic of acute congestive glaucoma. While the pupil may become oval, its orientation is typically vertical, not horizontal.
**Option C:** A circular pupil is not a typical finding in acute congestive glaucoma. The pupil may become irregularly shaped due to corneal edema, but a circular shape is not a characteristic feature.
**Option D:** A slit-like pupil is more characteristic of iridocyclitis or other conditions that cause pupil constriction, not acute congestive glaucoma.

**Clinical Pearl / High-Yield Fact**
In acute congestive glaucoma, a key clinical finding is corneal edema, which can be detected by examining the cornea for a hazy or cloudy appearance. Corneal edema is a critical factor in the diagnosis of acute angle-closure glaucoma.

**✓ Correct Answer: B. Oval and vertical**