Which of the following is a type of portable ladder?

A self-supporting ladder, commonly known as a folding or A-frame ladder, is designed to stand on its own without needing any external support. This type of ladder is widely used in various construction and maintenance tasks, as it provides stability and portability. The design allows it to be set up quickly and moved around easily, making it a versatile tool for workers.

In contrast, fixed ladders are attached to a structure and are not portable, while stationary ladders are similar in that they remain in place and cannot be moved easily. Adjustable ladders can change height but often require manual adjustment or additional equipment for support, which may not provide the same level of self-sufficiency as self-supporting ladders. Therefore, self-supporting ladders stand out as the best answer, emphasizing their unique design that allows them to be used independently in various settings.