The answer is option A.
Because the letter S has horizontal line of symmetry,
the letter I has both horizontal and vertical line of symmetry,
the letter N has vertical line of symmetry,
the letter A has vertical line of symmetry,
the letter O has both vertical and horizontal line of symmetry.
The answer is five because you have not specified about which line , horizontal or vertical ?
After this, the answer is also not five it is six because letter E has a horizontal line of symmetry. But we don't have option of 6 so we choose 5 (option A).