Answer
The reason why neutrophils, eosinophils, and basophils are all called granular leukocytes is that they all have granular vesicles which carry chemicals and are visible. These are visible when the cell is stained.
Work Step by Step
The explanation to this lies in the context of the word itself. "Granular" leukocytes contain granules or "vesicles" and "Agranular" ("a" meaning without) leukocytes are leukocytes that don't visibly contain these granular vesicles. When an agranular leukocyte is stained, you will not be able to see any "granules", because these granules aren't strong and are too small. Overall, an agranular leukocyte is named this way because there are no granules physically present when the cell is stained.
