The answer that best described the type of cellular transport that the student is investigating would be passive transport of osmosis: The correct option would be D.
To recall: osmosis is simply the movement of water molecules from the region of high water potential to the region of low water potential through a semi-permeable membrane.
From this definition:
The student observed the movement of water molecules from the beaker (high water potential) into the test tube (low water potential). Hence, active transport and diffusion are ruled out with osmosis being the only correct option.
More on cell transportation can be found here: https://brainly.com/question/23202309