Answer is A (Small intestinal biopsy): 'Small intestinal mucosal biopsy is essential in the evaluation of a patient with documented steatorrhea or chronic diarrhea (>3 weeks). -- Harrisons Failure to thrive in the background of recurrent bulky stools and abdominal pain suggests a diagnosis of chronic malabsorption. Although all options provided in the question may be used to suppo the diagnosis, the most appropriate investigation to make a specific diagnosis is small intestinal biopsy.