Which aspect can you manipulate to make a query activity more efficient?

Filtering dimension is the correct choice because it directly impacts the efficiency of a query activity by streamlining the records that need to be processed. When you set a filtering dimension, you narrow down the dataset based on specific criteria, which allows the system to quickly access and retrieve only the relevant records needed for your query. This minimizes the resource consumption and time required to execute the query, leading to improved performance.

In contrast, while adding more conditions might seem helpful in refining results, it can actually complicate the query process, potentially leading to longer processing times. Changing the data source can introduce additional overhead, as it could involve querying from a more complex or less optimized source. Modifying scheduling parameters, on the other hand, affects when the query runs rather than its efficiency in terms of data processing during execution. Thus, filtering dimension is the most effective way to enhance the efficiency of a query activity in Adobe Campaign Classic.