In the formula for the surface area of a triangular prism, \( S.A. = bh + (S_1 + S_2 + S_3)l \), the term "bh" represents the combined area of the two triangular bases.
This is because "b" is the base length of the triangle, and "h" is the height of the triangle, thus \( bh \) gives the area of one triangular base. Since there are two triangular bases in a prism, "bh" indicates the area of one base, and we can consider the total combined area of both bases in that context.
Therefore, the correct response is:
the combined area of the two triangular bases.