Reasonable diligence is the correct answer in this scenario.

Explanation:
Finding lost goods and returning them to their rightful owners can be a moral and legal obligation. However, if the true owner cannot be found despite reasonable efforts, the finder can realize the goods for themselves. This principle is based on the concept of "finders keepers" but with certain conditions.

Reasonable Diligence:
Reasonable diligence refers to the level of effort and care that a person should reasonably exercise in attempting to locate the true owner of lost goods. It involves taking reasonable steps to find the owner, such as checking for identification, asking around, notifying the authorities, or posting advertisements.

Importance of Diligence:
Diligence is important because it demonstrates that the finder has made a genuine effort to return the lost goods. It shows that the finder has acted responsibly and ethically in attempting to find the true owner. By exercising reasonable diligence, the finder can avoid any potential legal consequences or accusations of theft.

Other Options:
- Ordinary Diligence: Ordinary diligence may not be sufficient in this case as it implies a lesser level of effort compared to reasonable diligence. It may not meet the legal requirement of making a genuine attempt to locate the true owner.
- Lack of Diligence: Lack of diligence implies a complete absence of effort or care in finding the true owner. This would not be a valid reason to realize the goods, as it would be seen as negligence or theft.
- Due Diligence: Due diligence refers to the level of care and caution expected in a particular situation. While it is important to exercise due diligence, in this specific scenario, reasonable diligence is the standard to be met.

Conclusion:
In summary, a finder of lost goods can realize the goods for themselves if the true owner cannot be found despite reasonable efforts. Reasonable diligence involves making a genuine attempt to locate the owner and is the level of effort expected in such situations. By exercising reasonable diligence, the finder can fulfill their moral and legal obligations and avoid any potential legal consequences.