Which one of the following could be a clue that a property was formerly a waste disposal site?

The presence of soft spots, mounds, and discolored soil is a significant indicator that a property may have been previously utilized as a waste disposal site. These physical characteristics can suggest disturbances in the soil caused by waste materials beneath the surface. For instance, soft spots may indicate the presence of decomposing materials or contaminants that have altered the soil's structure, while mounds can be the result of earth being moved or piled up due to previous disposal activities. Discolored soil often points to chemical reactions involving waste materials that can change the color and texture of the earth.

In contrast, while the presence of birds might imply environmental conditions or food sources, it is not a definitive indication of a history involving waste disposal. A plate covering an oil fill area suggests different types of issues specific to oil storage rather than general waste disposal. Scorch marks on trees could imply fire damage or certain environmental factors, but they do not directly correlate to the previous use of the site as a waste disposal location. Thus, the combination of soft spots, mounds, and discolored soil serves as a multifaceted clue that points more conclusively toward a past as a waste disposal site.