The correct answer is: B. shuffle
The shuffle stage is responsible for grouping the Reducer inputs by key. This is done by sorting the intermediate data by key and then partitioning it into smaller chunks. This allows the Reducer to process the data in parallel.
The sort stage is responsible for sorting the intermediate data by key. This is done to ensure that the Reducer receives the data in a consistent order.
The reduce stage is responsible for reducing the intermediate data to a smaller set of values. This is done by applying a function to each group of data.
None of the mentioned is not the correct answer because the shuffle stage is responsible for grouping the Reducer inputs by key.