Answer: DataFrame.groupBy()

A GroupedData object is returned after grouping operations in Spark, which allows subsequent aggregations. The correct methods that return GroupedData are groupBy(), cube(), rollup(), etc. Among the options provided, only option D (DataFrame.groupBy()) is the correct method that returns a GroupedData object. The other options either do not exist in Spark's DataFrame API or are not methods that return a GroupedData object.

Author: LeetQuiz Editorial Team