Orthogonal lines are A. the point(s) to which all objects in a picture lead.
B. the converging lines that lead to one or more vanishing points.
C. where the viewer sees the landscape.
D. the flat surfaces of a drawing, painting, or a relief.

B.the converging lines that lead to one or more vanishing points.

Explanation:

When drawing an object, we have to put them into the right perspective. To be sure they are placed properly to create the illusion of existing in the real space, we can check by drawing orthogonal lines from the corners of the object. These are the imaginary lines that do not exist in real life, but that we project to make sure the perspective is right. The lines should meet up at one vanishing point in the distance.

When we do draw orthogonal lines, we should do so lightly as they are not the existing element on the drawing.