Introduction:
A space truss is a three-dimensional structure composed of interconnected bars or members. It is used in various engineering applications, including aerospace, civil, and mechanical engineering. The simplest element of a space truss refers to the basic building block or unit from which the entire truss structure is constructed.

Explanation:
The simplest element of a space truss is a tetrahedron, which is represented by option B. A tetrahedron is a polyhedron with four triangular faces, six edges, and four vertices. It is the simplest geometric shape that can form a three-dimensional structure.

Reasoning:
To understand why a tetrahedron is the simplest element of a space truss, let's compare it with the other options:

1. Triangle (Option A):
- A triangle is a two-dimensional shape with three sides and three angles.
- In a space truss, triangles alone cannot form a three-dimensional structure as they are confined to a single plane.
- A space truss requires three-dimensional elements to provide stability and load-bearing capacity.

2. Tetrahedron (Option B):
- A tetrahedron is a three-dimensional shape with four triangular faces, which are connected at their vertices.
- It is the simplest polyhedral element that can form a three-dimensional truss structure.
- Tetrahedrons can be connected at their vertices to create a network of interlocking units, forming a space truss.
- The interconnected tetrahedrons provide stability and strength to the overall truss structure.

3. Octahedron (Option C):
- An octahedron is a three-dimensional shape with eight triangular faces.
- While an octahedron can also form a three-dimensional structure, it is more complex than a tetrahedron.
- Octahedrons require more connections and additional members compared to tetrahedrons to create a stable truss structure.

4. Pyramid (Option D):
- A pyramid is a three-dimensional shape with a polygonal base and triangular faces converging to a single vertex.
- Pyramids can be used in truss structures, but they are more complex than tetrahedrons.
- Pyramids require additional members and connections compared to tetrahedrons, making them less simple in terms of the truss design.

Conclusion:
In summary, the simplest element of a space truss is a tetrahedron. It is the most basic three-dimensional shape that can form a stable and load-bearing truss structure. The interconnected tetrahedrons create an efficient and visually appealing design, providing strength and stability to the overall truss system.