Answer: The widest stamp is the one with a 1 3/4 width.
Explanation:
Compare all of these widths by converting the fractions to the same denominator of 16:
1 5/8 = 1 10/16
1 11/16 = 1 11/16
1 3/4 = 1 12/16
1 1/2 = 1 8/16
As noted above, the widest width is 1 3/4 (1 12/16)