A. w3-border-top
B. 3-border-right
C. w3-border-bottom
D. w3-border-gutter

Solution

D. w3-border-gutter

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