What could a dull and grainy appearance of solder indicate?

A dull and grainy appearance of solder can be an indication that it has potentially overheated during the soldering process. When solder overheats, it can lose its shiny and smooth appearance, becoming dull and grainy instead. This can occur due to prolonged exposure to high temperatures or excessive heat, which can negatively affect the solder's properties and its ability to create a reliable electrical and mechanical connection in a joint.

In contrast, a shiny appearance typically signifies good solder quality, indicating a successful joint. The other options do not align with the characteristics associated with solder that appears dull and grainy, further reinforcing that overheating is a plausible cause for this visual defect.