CD56 is most closely associated with which type of leukocyte?

CD56 is a surface marker primarily associated with natural killer (NK) cells, which are a critical component of the innate immune system. NK cells play a vital role in the body's defense against tumors and viral infections by recognizing and destroying infected or malignant cells. The presence of CD56 is a distinguishing feature used to identify NK cells among other lymphocyte populations.

In contrast, neutrophils do not express CD56; they are identified by other markers such as CD15 and CD16. B cells are characterized by the presence of CD19 and CD20, while T cells typically express CD3 and can be further specified by either CD4 or CD8. Therefore, CD56's association is definitive for NK cells, making it the correct choice for this question.