Final answer:
Basophils are the leukocytes that are not commonly seen on many blood smears due to their rarity and the large, obscuring granules they contain.
Step-by-step explanation:
The leukocytes not commonly seen on many blood smears are basophils. Basophils are the least common leukocytes, typically comprising less than one percent of the total leukocyte count. They are characterized by large granules that pick up a dark blue stain and sometimes these granules may obscure the two-lobed nucleus, which makes basophils more difficult to detect under a microscope. Unlike basophils, neutrophils, lymphocytes, and monocytes appear more frequently on blood smears. Therefore, the correct answer to the question of which leukocytes are not seen at all on many blood smears is D) Basophils.