Answer: Their supporting structure would be most likely the MUSCULOSKELETAL SYSTEM of the multicellular organisms.
Step-by-step explanation:
The musculoskeletal system found in multicellular organisms are made up of structures which provides support, protection and allows for various movements. These structures include:
--> Muscles
--> Cartilage
--> Ligaments
--> Tendons and
--> Joints
Furthermore, Microtubules and microfilaments are the major components of cytoskeleton. They are the structures that gives a cell it's shape and helps organise it's parts. As described in the question, they are elongated structures that cross the cell, giving SUPPORT to the cell, and carries organelles and objects throughout the cell.
Therefore, since the Microfilaments and Microtubules provides support and shape at the cellular level it can be most likened to the musculoskeletal system of multicellular organisms which performs the same functions.