Mary’s discovery that the cell membrane of a bacteria acts like a fence because it keeps some things in and other things out is best categorized as an analogy.
Here’s why: an analogy is a comparison between two different things that highlights some form of similarity. In this case, Mary compares the function of the cell membrane to a fence, which is a familiar concept. A fence clearly serves to protect and separate, allowing only certain things in or out, much like the selective permeability of a cell membrane.
An observation would simply note that bacteria have a cell membrane without interpreting or comparing its function. An inference would be an assumption drawn from evidence or reasoning, which isn’t what Mary is doing here. By using the concept of a fence to illustrate the role of the cell membrane, she is employing an analogy to clarify its function.