Complex transport tubes which move water, nutrients, and sugar throughout some plants belong to the organ level of organization.

Here's why:

* Tissue: A tissue is a group of similar cells that work together to perform a specific function. Xylem and phloem, the transport tubes, are examples of tissues.

* Organ: An organ is composed of different tissues working together for a common purpose. In this case, the xylem and phloem, along with other tissues like parenchyma cells, make up the vascular system, an organ responsible for transport within the plant.

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