The most effective strategy for managing old data in Delta Lake, especially when compliance and cost optimization are priorities, is to implement a data retention policy based on a time-based column such as a timestamp. This approach allows for the systematic identification and processing of records that exceed the defined retention period, ensuring that only relevant data is retained. Option A is incorrect because random selection does not guarantee compliance with retention policies or efficient storage use. Option B is flawed because unique identifiers do not necessarily correlate with the age of the data. Option D is inappropriate as it focuses on access patterns rather than data age, which does not address the requirement for archiving or deleting old data.