In a triangular prism, the cross-sections parallel to the bases (triangular faces) are triangles, while the cross-sections that are parallel to the rectangular faces are rectangles.
If Mateo has drawn a line to represent the parallel cross-section of the triangular prism along the length of a rectangular face, it means that he is indicating a cross-section that is parallel to the rectangular faces.
Thus, the correct statement would be: Yes, the line should be parallel to one of the rectangular faces.
This is because cross-sections taken parallel to the rectangular faces will create rectangular cross-sections, while cross-sections taken parallel to the triangular faces will create triangular cross-sections.