Final answer:
Based on the given sizes, where the plant cell's diameter is 1.26 µm and the bacterium's length is 5.1 µm, the bacterium is larger. Typically, eukaryotic cells are larger than prokaryotic ones, which suggests a possible error in the question's data.
Step-by-step explanation:
To compare the diameters of the plant cell and the bacterium from the given information, we must recognize that plant cells are generally eukaryotic, and bacterial cells are prokaryotic. According to details on cell size, prokaryotic cells are smaller than eukaryotic cells, with diameters ranging from 0.1 to 5.0 µm for prokaryotic cells and 10 to 100 µm for eukaryotic cells.
Given that the diameter of the plant cell is 1.26 µm and falls below the typical range for eukaryotic cells, there might be an error in the question. However, if we take the figures at face value, comparing it to the length of the bacterium which is 5.1 µm, we would conclude that the bacterium's diameter (or length, as per the original question, which might suggest its longest dimension) is larger than the diameter of the plant cell. Thus, the correct answer, based on the information provided, would be (b) The bacterium diameter is larger than the plant cell.