The correct answer is D) decrease list or trim. When a vessel experiences a severe list or trim due to off-center ballast, counter-flooding into empty tanks will help restore the vessel's equilibrium and decrease the list or trim. Counter-flooding shifts the vessel's center of gravity back towards the centerline, which increases the righting moment and allows the vessel to regain its upright position. The other options are incorrect because: A) increasing list or trim would worsen the situation, B) increasing the righting arm does not directly address the off-center ballast issue, and C) while increasing the righting moment can help, the key is to decrease the list or trim, which is what counter-flooding achieves.