To answer this question, we need to understand the process of writing a sequential file using a Sequential file stage in the parallel engine.
When a sequential file is written using a Sequential file stage, the parallel engine inserts an operator to convert the data from internal format to the external format. The internal format refers to the format in which the data is stored and processed within the parallel engine, while the external format refers to the format in which the data is written to the sequential file.
The operator that is inserted to perform this conversion is the Export operator. The Export operator takes the data in the internal format and converts it into the appropriate external format before writing it to the sequential file.
Therefore, the correct answer is:
A. Export operator - This option is correct because the Export operator is inserted by the parallel engine to convert the data from internal format to the external format before writing it to the sequential file.