The correct answer is option 'B' - Square.

Square Pyramid:
A square pyramid is a type of pyramid where the base is a square. It is a three-dimensional geometric shape with a square base and four triangular faces that meet at a single point called the apex or vertex. The height of the pyramid is the perpendicular distance from the apex to the base.

Explanation:
To understand why the correct answer is option 'B' - Square, let's break down the pyramid and its properties:

1. Base:
- The base of a pyramid is the bottom face upon which the pyramid stands.
- In the case of a square pyramid, the base is a square.
- A square is a quadrilateral with all four sides equal in length and all four angles equal to 90 degrees.

2. Faces:
- A pyramid has triangular faces that converge to a single point called the apex.
- In the case of a square pyramid, it has four triangular faces.
- Each face of the pyramid is a right-angled triangle, where one side is the height of the pyramid, and the other two sides are the slant height of the pyramid.

3. Apex:
- The apex is the topmost point of the pyramid where all the triangular faces meet.
- In the case of a square pyramid, the apex is directly above the center of the square base.

4. Height:
- The height of a pyramid is the perpendicular distance from the apex to the base.
- In the case of a square pyramid, the height is the distance from the apex to the center of the square base.
- The height is not necessarily equal to the slant height of the pyramid or the length of its sides.

Therefore, based on the definition and properties of a square pyramid, we can conclude that a square pyramid has a square as its base.