Explanation
w3css provide following classes for border
- w3-border
- w3-border-top
- w3-border-right
- w3-border-bottom
- w3-border-left
- w3-border-0
- w3-border-color
- w3-hover-border-color
- w3-bottombar
- w3-leftbar
- w3-rightbar
- w3-topbar
so w3-border-gutter is correct answer