Final answer:
The cell described as having a rounded or slightly indented nucleus filling most of the cell is the mature lymphocyte, responsible for the adaptive immune response.
Step-by-step explanation:
The cell with a rounded or slightly indented nucleus that almost completely fills the cell is the mature lymphocyte. Lymphocytes are agranulocytes, including B and T cells, which are responsible for the adaptive immune response. Unlike granulocytes such as neutrophils, eosinophils, and basophils which have a lobed nucleus and granular inclusions, lymphocytes have a large, often rounded nucleus with little cytoplasm, distinguishing them from other white blood cell types.