Salesforce JavaScript Developer Practice Exam

The syntax for declaring a reactive variable in Lightning Web Components (LWC) is through the usage of the `@track` decorator. This decorator is utilized to observe changes to the variable's value and to automatically re-render the component when the variable is updated. By prefixing the variable declaration with `@track`, developers can ensure that any changes to the variable will trigger a reactivity in the UI, allowing the component to update automatically to reflect the current state of the `myVar`.

Using this syntax, when `myVar` is set to a new value, any bound UI elements will reactively update to show the new value. This makes `@track` essential for handling complex data structures and ensuring smooth user interactions within the component.

The other options do not accomplish the same goal of establishing reactivity in the component context. For instance, declaring a variable with `let` or `var` does not inherently track its changes or trigger updates. While importing `track` from LWC is a prerequisite for using it, it does not itself define a reactive variable.