Which of the following is a characteristic feature of malignant tumors?

Malignant tumors are typically characterized by their irregularly formed masses. Unlike benign tumors, which tend to be well-defined and encapsulated, malignant tumors exhibit an uncontrolled growth pattern that leads to an irregular appearance. This irregularity is indicative of their invasive nature, as they can penetrate surrounding tissues and disrupt normal cellular architecture. Such tumors may also have a heterogeneous composition, which means they can vary in structure and density due to the rapid and disorganized growth of cancerous cells.

In contrast, the other characteristics mentioned do not accurately define malignant tumors. For instance, encapsulated tumors are often benign, leading to a well-defined boundary that is not present in malignant tumors. Furthermore, nonmalignant by definition refers to conditions or tumors that are not cancerous, and slow-growing tumors can also describe many benign tumors or certain low-grade malignancies, but they do not represent the typical behavior of malignant tumors which are often fast-growing and aggressive. Thus, the irregularity in formation is a hallmark that illustrates the aggressive and invasive nature of malignant tumors.