Which of the following terms means “a decrease in cell number” in medical terminology?

The term that signifies “a decrease in cell number” is indeed indicated by the suffix "-penia." This suffix is commonly used in medical terminology to describe conditions characterized by an abnormally low number of a specific type of cell. For instance, conditions like leukopenia refer to a decreased number of white blood cells, while thrombocytopenia indicates a low platelet count.

The other options represent different medical concepts:

"-osis" generally denotes a pathological condition or disease, often implying an increase rather than a decrease, as seen in terms like leukocytosis (an increase in white blood cells). "-emia" refers to a condition related to the blood, often signifying the presence of a substance or a specific count in the blood, such as anemia, which indicates a lack of red blood cells. Lastly, "-oma" is used to denote a tumor or neoplasm, which often implies an abnormal growth rather than a reduction. Thus, the usage of "-penia" is specifically appropriate for indicating a decrease in cell counts.