What class is required for all column containers in Bootstrap?

The requirement for all column containers in Bootstrap is to use the class 'portlet-column'. This class is integral to the Bootstrap grid system, which is designed to create responsive layouts that adapt to various screen sizes. When building an interface using Bootstrap, using the appropriate classes allows developers to define how columns behave within a row. The 'portlet-column' class specifically indicates that an element is part of a column layout, enabling better alignment and spacing in conjunction with other Bootstrap utilities.

The other options do not correspond with Bootstrap's established class system for creating columns. While Bootstrap has a variety of classes for layouts, 'portlet-column' stands out as the key class to denote a column container specifically within Bootstrap's framework. Understanding this concept is essential for building well-structured and responsive web applications using Bootstrap.