Which cocktail includes gin and a splash of soda water?

The cocktail that includes gin and a splash of soda water is the Tom Collins. This classic drink is made with gin, fresh lemon juice, sugar, and topped with soda water, creating a refreshing and effervescent beverage. The addition of soda water not only lightens the drink but also balances the tartness of the lemon juice and the sweetness of the sugar, resulting in a well-rounded flavor. The use of gin provides a botanical character that is distinct to the Tom Collins, making it a quintessential choice among gin cocktails.

In contrast, the other options do not feature gin or soda water as key ingredients. The Midori Sour primarily utilizes Midori melon liqueur and does not include gin. The Whiskey Sour is based on whiskey and lemon juice, while the Salty Dog is typically made with vodka or gin and grapefruit juice, without the characteristic addition of soda water. Thus, the combination of gin and soda water uniquely identifies the Tom Collins among the given choices.