The keyword used to define the branch that is executed when all previous conditions are false is "else." In programming, the "else" statement is part of a conditional structure that allows the code to handle scenarios when none of the preceding "if" or "else if" conditions apply.
When you write a series of conditional statements, you start with an "if" that checks a specific condition. If that condition is false, you might have one or more "else if" statements to check additional conditions. However, if all of those conditions are false, the "else" block will execute, allowing for a default or catch-all behavior in your code. This is particularly useful for providing a fallback outcome when no other conditions are met, thereby ensuring that your program can respond appropriately under various circumstances.
The other keywords listed serve different purposes: "if" is used for initial condition checks, "else if" allows you to evaluate further conditions if the preceding "if" is false, and "switch" is typically employed for selecting among several options based on a single expression's value. None of these options fulfil the role that "else" plays in terms of providing a default execution path.