Final answer:
The thickest lines on an engineering drawing are outline or object lines, which represent the edges of the object being drawn.
Step-by-step explanation:
In engineering drawings, the thickest lines, often referred to as outline or object lines, represent the edges of an object. These lines are thick and dark to clearly distinguish the shape and end points of the parts of the design. In contrast, dimension lines, extension lines, and section lines tend to be thinner, as their primary functions are to display dimensions, extend from a point to the dimension line, and show the material cut through, respectively. Center lines are also used in engineering drawings to indicate axes of symmetry and are typically drawn as long and short dashed lines.
Contour lines may also appear in engineering drawings when representing topographical maps and indicate areas of the same elevation. These are not necessarily the thickest lines but are crucial for understanding the topography in civil engineering designs.