In a triangle, the height is the perpendicular line from a vertex to the opposite side (the base). The base is the side that is perpendicular to the given height.<br><br>
&bull; From the figure: AX is drawn as a perpendicular line from vertex A.<br>
&bull; AX is perpendicular to side BC.<br>
&bull; Therefore, if the height is AX, the base is BC.<br><br>
Final Answer: B: BC