Most likely a metamyelocyte or monocyte..
Of course, "Kidney-bean-shaped" is a pretty broad description, and many cells could have that vague appearance on a slide. Most commonly that description is associated with metamyelocytes, which are immature Granulocytes (Segs, Eos, and Basos). However, if the question was in reference to a fully matured WBC, it likely would be a monocyte.
If you are trying to tell them apart, remember monocytes (like lymphs) are not Granulocytes, and should not have cytoplasmic granules. A meta will (usually) have granules.
monocyte
erothyctes
Basophils, Eosinophils,& Neutrophils (BEN)
Lymphocytes
leukocyte
The bacteria uses the nucleus
lymphocyte, specifically b-cells source: anatomy book
Lymphocyte
Basophil is the type of leukocyte that contains heparin and is an anticoagulant. The type of leukocytes that become macrophages in the tissues are monocytes.
erothyctes
lymphocytes
neuthrophils
Basophils, Eosinophils,& Neutrophils (BEN)
PMN: polymorphonuclear leukocyte because its nucleus have many lobes
Granular Leukocytes account for about 50-70% of all WBCs, and include neutrophils, eosinophils, and basophils. Agranular Leukocytes account for 25-35% of all WBCs. They include lymphocytes and monocytes. Apart from the difference in function regarding each individual type of white blood cell, the primary difference between granular and agranular leukocytes is that the former has visible granules, whereas the later does on have any noticeable granules.
Lymphocytes
Basophil :)
cell membrane