The term "single malt whisky" specifically refers to whisky produced at a single distillery using only malted barley as the grain. This means that all the whisky in the bottle comes from one distillery's production and is made exclusively from malted barley, which is a key ingredient in the production of this style of whisky.
Single malt whisky is renowned for its unique flavors and characteristics that reflect the climate, production methods, and local ingredients of the distillery. This definitiveness in sourcing and production process distinguishes single malt whisky from other types, such as blended whiskies that may incorporate whiskies from multiple distilleries or grains other than barley.
The other options do not accurately capture the essence of what defines "single malt whisky." For instance, whisky made from multiple malted grains falls under the category of blended or grain whisky, which does not align with the single-malt standard. Whisky blended from different regions refers to a mix of whiskies from various locations, clearly differing from the single-origin requirement of single malts. Finally, the aging process in various barrels can apply to many types of whisky and is not a defining feature of single malt. Thus, the focus on a single distillery is what makes the designation of "single malt whisky