Which type of RTE cells are characterized by being cuboidal and never appearing round?

The correct answer is that collecting duct RTE cells are characterized by being cuboidal and never appearing round. Collecting duct epithelial cells are typically cuboidal in shape due to their specific role in the reabsorption of water and electrolytes, as well as acid-base balance in the kidney. Their cuboidal appearance allows for the formation of tight junctions between cells, which is crucial for the proper functioning of the collecting ducts. This structure helps maintain the necessary gradient for water reabsorption and the concentration of urine.

Proximal convoluted tubule cells, while also having a cuboidal shape, can vary in appearance and are involved in the reabsorption of a significant portion of filtered solutes and water. Distal convoluted tubule cells are generally more irregular and may have a different appearance that can include rounded shapes. Squamous cells, on the other hand, are flat and are commonly found in areas where passive diffusion is required, such as in the lining of the Bowman's capsule and the bladder, thus not fitting the criteria given in the question. Therefore, the characteristic cuboidal shape of collecting duct RTE cells is what makes them the correct answer in this context.