Which of the following represents a valid defense against the charge of trespassing for Sally?

The valid defense against the charge of trespassing for Sally is that she was unaware she was on private land. Trespassing typically involves both a physical invasion of someone else's property and the intent to be there, or at least knowledge that it is private property. If Sally genuinely did not know she was on private land, it indicates a lack of intent to trespass. In the eyes of the law, the absence of intent can often serve as a valid defense, as trespassing often requires a knowing and willful act. Therefore, if Sally was not aware she was trespassing, it can be argued that she did not commit the offense at all.

In contrast, the other options present issues related to consent or knowledge:

• Permission from the landowner's grandfather may not hold because the grandfather’s authority to grant permission may not be legally recognized if he no longer owns the property or does not have authority to permit entry.

• An invitation by another visitor does not create a defense unless that visitor has the authority to grant access to the property, and typically, visitors cannot grant permission for others to enter a private property.

• Having been on the property before does not provide a legal defense, as previous presence does not imply current permission or an excuse for entering