The correct option is D He fixed metal pins.
Option D: As it's mentioned in the line: "To make the pair last longer, Jacob decided to fix small metal pins or rivets near the pockets, zipper and other areas. The overalls were tough and looked strong" the technique is mentioned in option D. Hence it's the correct option.
Options A, B and C are not mentioned as techniques to make the overalls stronger. Hence they're incorrect.