Which option best describes a tumor?

A tumor is best described as an abnormal mass that can vary significantly in size, shape, and color. This definition encompasses both benign and malignant growths, which can arise from various tissue types within the body. Tumors can be solid or fluid-filled and may develop as a result of an uncontrolled proliferation of cells, resulting in an abnormal structural formation.

In contrast to other options, a small blister containing fluid is indicative of a localized reaction to irritation or injury rather than a growth or mass. An itchy lesion caused by allergies would more commonly be described as a rash or dermatitis, not a tumor. Similarly, a flat, colored spot on the skin could refer to a mole or pigmentation change, but it does not encompass the variability in growth characteristics that tumors may exhibit. The distinguishing features of a tumor as an abnormal, evolving mass make this characterization crucial in both medical terminology and practice.