4. The first of the many agreements that were agreed on by the five trading partners proved to be the most satisfactory.

A) that were agreed on by the five trading partners proved

B) that was agreed on by the five trading partners proved

C) that were signed by the five trading partners proved

D) that the five trading partners agreed on proved

E) that the five trading partners signed proved

[spoiler]IMO : D
OA: E[/spoiler]