Bootstrap's grid system is fundamental to responsive web design, allowing developers to create flexible layouts that adapt to different screen sizes. This ensures a consistent and visually appealing presentation across devices.

Nested grids in Bootstrap enable the creation of multiple levels of grid structures within a page layout. This is useful when you need to organize content into more complex arrangements. To implement nested grids, you can simply include additional grid structures (rows and columns) within the columns of the parent grid. This way, you can achieve a more intricate and flexible layout design.

In Bootstrap, the primary method to customize a theme's color scheme is to override default styles with custom CSS. This allows you to tailor the colors according to your design preferences without directly modifying the Bootstrap files.

The 'viewport' meta tag is essential in Bootstrap for achieving responsiveness on mobile devices. It sets the viewport width and initial scale for proper rendering.

To customize the behavior of a Bootstrap dropdown using JavaScript, you can use event listeners to capture the 'show' and 'hide' events. This allows you to add your custom logic to modify the dropdown's behavior based on these events. For example, you can dynamically change the dropdown content or prevent it from closing under certain conditions.

The correct option is @import. In SASS, the @import directive is commonly used to include external SASS files, allowing developers to import Bootstrap's SASS files and override variables before importing Bootstrap styles. This is a powerful feature for customizing Bootstrap styles to suit specific project requirements.

The 'text-truncate' utility in Bootstrap is used to automatically truncate long text within a container and add an ellipsis (...) to indicate that the text has been cut off. This is particularly useful when dealing with limited space or fixed-size containers in web design.

The Cache-Control header is commonly used to control caching behavior. By setting appropriate values, such as max-age, you can instruct browsers to cache resources, improving performance by reducing the need to download them repeatedly.

Event delegation in Bootstrap Tooltips involves binding the tooltip events to a static parent container. This allows dynamically created elements within the container to inherit the tooltip behavior without individually attaching events to each element.

Asynchronous or deferred loading of Bootstrap JavaScript files allows scripts to load independently, improving page load efficiency by loading in parallel. This prevents scripts from blocking each other, enhancing overall performance.