The correct answer is: the volume of the pyramid is one-third the size of the volume of the prism.
To clarify: the volume of a prism is given by the formula \( V = B \cdot h \), where \( B \) is the area of the base and \( h \) is the height. The volume of a pyramid is given by the formula \( V = \frac{1}{3} B \cdot h \), where \( B \) is again the area of the base and \( h \) is the height.
Therefore, if a prism and a pyramid have congruent triangular bases and the same heights, the volume of the pyramid will be one-third that of the prism.