Final answer:
The leukocyte observed through the microscope with a U-shaped nucleus and no visible granules is a B) monocyte.
Step-by-step explanation:
Through the microscope, if you observe a very large leukocyte with a U-shaped nucleus but no granules, you are likely looking at a B) monocyte.
A monocyte is an agranular leukocyte, meaning its granules are much fewer and less visible compared to granular leukocytes such as neutrophils, eosinophils, and basophils. Monocytes have a distinct simple-shaped nucleus that might display some indentation but is not lobed, and they can differentiate into macrophages and dendritic cells, which are essential for the body's immune response.