The correct answer is C) Lack of penetration. Lack of penetration is a weld fault that can only be detected by a method that examines the internal structure of the weld, such as radiographic or ultrasonic testing. This is because lack of penetration occurs when the weld does not fully fuse with the base metal, creating an internal discontinuity that is not visible on the surface. The other options, such as lack of reinforcement, undercut, and overlap, are surface defects that can be detected through visual inspection or other non-destructive evaluation methods. These defects are typically more easily identified without the need for internal examination of the weld.