Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)2, in water. It is commonly used as an antacid to neutralize excess stomach acid because it is a mild base. When ingested, it reacts with hydrochloric acid in the stomach to form magnesium chloride and water, thereby relieving heartburn and indigestion.

Alkalis are basic in nature and are characterized by a bitter, astringent taste and a soapy, slippery feel when touched. They are known to be corrosive to skin and materials. Sour taste is a defining characteristic of acids, not alkalis (bases). Therefore, stating that alkalis are sour is factually incorrect.

Limestone, chalk, and marble are all forms of calcium carbonate (CaCO3), which reacts with HCl to release CO2. Quicklime is calcium oxide (CaO). When CaO reacts with HCl, it undergoes a neutralization reaction to form calcium chloride (CaCl2) and water (H2O), but it does not produce carbon dioxide gas.

Muriatic acid is a historical and industrial name for hydrochloric acid (HCl). It is commonly sold in hardware stores for cleaning masonry, adjusting the pH of swimming pools, and various other industrial cleaning applications. The term 'muriatic' is derived from the Latin word 'muria', meaning brine or salt.

Aqua regia is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a 3:1 molar ratio by volume. This mixture is famous for its ability to dissolve noble metals like gold and platinum due to the formation of nitrosyl chloride and nascent chlorine, which act as powerful oxidizing agents.

Statement B is incorrect because the reaction between an acid and an alkali (base) is called a neutralization reaction, not a decomposition reaction. Neutralization involves the formation of salt and water. The other statements (A, C, and D) correctly describe the standard chemical reactivity of acids with metal oxides, active metals, and carbonates respectively.

Curd is produced by the fermentation of milk by lactic acid bacteria, primarily Lactobacillus. These bacteria convert lactose into lactic acid, which lowers the pH and gives curd its characteristic sour taste. Note: The provided answer key indicates C (acetic acid), which is factually incorrect as acetic acid is found in vinegar. Lactic acid is the correct chemical component.

Hydrochloric acid (HCl) is a binary acid consisting of hydrogen and chlorine, containing no oxygen atoms. In contrast, oxyacids like nitric acid (HNO3) and sulphuric acid (H2SO4) contain oxygen atoms bonded to the central atom. Therefore, HCl is the correct choice.

Sodium chloride (NaCl) is formed through the neutralization reaction between hydrochloric acid (HCl), which is a strong acid, and sodium hydroxide (NaOH), which is a strong base. Because both components are strong, the resulting salt is neutral in aqueous solution.

The statement that acids have a salty taste is incorrect; acids are characterized by a sour taste. Strong acids are defined by their complete dissociation in aqueous solutions, releasing a high concentration of hydrogen ions. Common examples include HCl, H2SO4, and HNO3, which are highly reactive.