Explanation:

When we talk about the vertical cut of a brick, we are referring to the cross-section that is obtained when a brick is cut vertically. This means that the cut is made perpendicular to the base of the brick and parallel to the length of the brick.

Understanding the Cross-Section:
The cross-section of an object is the shape that is obtained when it is cut straight across. In the case of a brick, the cross-section will reveal the shape of the interior of the brick.

Properties of a Brick:
To understand why the cross-section of a brick is a rectangle, let's consider the properties of a brick:

1. Shape: A brick is a rectangular prism, which means it has six faces, all of which are rectangles.
2. Dimensions: A standard brick has a length, width, and height. The length and width are usually larger than the height.

Vertical Cut:
When a vertical cut is made on a brick, it is made perpendicular to the base of the brick. This means that the cut is made from the top to the bottom of the brick, dividing it into two equal halves.

Cross-Section Shape:
Since the brick is a rectangular prism, when it is cut vertically, the cross-section will be a rectangle. The cut will reveal the rectangular shape of the interior of the brick.

Visual Representation:
To visually represent the cross-section of a brick, imagine looking at the cut surface of the brick from the side. You will see a rectangular shape.

Therefore, the correct answer is option 'C' - rectangle.