The correct summary is:
A parallelogram has two lines of reflection that carry it onto itself: one horizontal through the midpoints of each side, and one vertical through the midpoints of the top and bottom.
However, note that a parallelogram also has two additional lines of reflection along its diagonals, which means it actually has four lines of symmetry in total: horizontal, vertical, and both diagonals. The best option that features this complete set of symmetries would be:
A parallelogram has four lines of symmetry: horizontal through the midpoints, vertical through the midpoints, and both diagonals.
So, if the focus is strictly on lines of reflection that carry it onto itself, the diagonal reflections should also be included.