The correct operation to return the number of rows in a DataFrame is DataFrame.count(). Option D is correct because count() is the standard Spark DataFrame action that triggers computation and returns the total row count.

• A: numberOfRows() is not a valid DataFrame method in Apache Spark.
• B: n() is not a recognized method for counting rows in Spark.
• C: sum() aggregates numerical columns and does not count rows.
• E: countDistinct() is used for counting distinct values in columns, not total rows. Thus, only option D is correct.