Correct Answer - a
(a) If sea water is drunk, its Mg^(2+) ions incraese the solute concentration in the intestinal lumen because `Mg^(2+)` is absorbed very slowly. On the contrary, `Mg^(2+)` draws water from the blood to the intestinal lumen by osmosis. So, water is not gained, but is lost from the blood on drinking sea water. Thus, there occurs increase in the fluidity and volumes of intestinal contents, in the same way this consequently stimulates intestinal paristalsis and evacuation of fluid faeces.