Concept:An antacid is a weak base that neutralises excess acid (hydrochloric acid) in the stomach and relieves stomach upset (indigestion).Solution:Mg(OH)2 is magnesium hydroxide, a weak base.A suspension of magnesium hydroxide in water looks white like milk, so it is sold as milk of magnesia.It neutralises stomach acid: Mg(OH)2+2HCl→MgCl2+2H2OMilk of aluminium is not a common name; trisilicate (magnesium trisilicate) is a different antacid with a different formula; Martins liver salt is a different product.So the first aid labelled Mg(OH)2 is milk of magnesia.Answer:Option B (Milk of magnesia)