A patient taking omeprazole, if he stops taking it, will be prone to:

Omeprazole (a proton pump inhibitor) is used to treat/prevent conditions caused by excess stomach acid, such as peptic ulcer disease and GERD. Stopping the medication removes that acid suppression, making the patient prone to recurrence/worsening of peptic ulcer disease (the opposite of D, since stopping the PPI would mean stomach acid production returns toward normal/elevated, not decreased).