**Category:** Geometry  
**Subtype:** Solid Geometry (specifically, pyramids)  
**Problem Description:** The problem involves a pyramid (specifically a tetrahedron) with a triangular base. It asks for the angle between the line segment from the apex of the pyramid (point S) to one of the base vertices (point B) and the base plane. The base is an equilateral triangle with side length \(a\), and the height from the apex to the base is also \(a\). The options provided are different angles to choose from.