Which of the following may cross the membrane of a cell unassisted?

The correct answer is (B) Testosterone.

Testosterone is a steroid hormone, which is lipid-soluble. This enables it to easily cross the cell membrane without the need for assistance. In contrast, glucose is a polar molecule and typically requires transport proteins to move across the membrane, and sodium ions also need specific channels or transporters due to their charge. If you have more questions or need further explanations, feel free to ask!