What type of ladder is designed to be used at varying heights and can be extended?

An extension ladder is specifically designed to be adjustable in height, making it suitable for reaching various elevation points. This type of ladder consists of two or more sections that can slide past one another, allowing the user to extend or retract the ladder to achieve the desired height. This versatility is particularly useful for tasks that require access to higher areas, such as roofs or upper floors, while providing the necessary stability and safety when extended properly.

The other types of ladders have specific uses that do not include the ability to extend. A single ladder is typically a fixed height and cannot be adjusted, while a platform ladder is designed for stability and working at a set height rather than being extendable. A step ladder, on the other hand, is designed for low-level tasks and features a fixed height with a platform to stand on but does not extend. Therefore, the extension ladder stands out as the best option for varying heights due to its extendable nature.