The correct option is
D insignificant
The word 'cardinal' refers to something of the greatest importance or fundamental.
Option A, minor refers to something of lesser importance, significance or, seriousness.
Option B, vital refers to something absolutely necessary.
Option C, debatable refers to something questionable or disputable.
Option D, insignificant refers to something too small or unimportant to be worth consideration.
Option E, avoidable refers to something that is needless or preventable.
Thus, according to the meaning, the word 'insignificant' is the opposite of the word 'cardinal'.