After an index is created in HyperSpace, several actions can be performed on it like Refresh the index if the underlying data changes
Delete if the index is not needed
Vacuum if an index is no longer needed
What do you mean by vacuum index?

Answer options:

A.Delete Only the content of the index
B.Restructure the index
C.Physical deletion of the index contents and related metadata completely from Hyperspace`s metadata.
D.Only remove the data while preserving the metadata
E.Update the index

Correct Answer: C 
Vacuuming an index means performing a hard delete i.e. removing files and the metadata entry completely for that index using the vacuumIndex command.Option A is incorrect. Vacuum the index does not mean to delete only the content.
Option B is incorrect. Vacuum the index does not mean to restructure the index.
Option C is correct. Vacuum the index means physical deletion of the index contents and related metadata completely from Hyperspace`s metadata.
Option D is incorrect. Vacuum the index deletes not only data but also metadata.
Option E is incorrect. Updating the index is not the right answer.
Reference:
To know more about HyperSpace, please visit the below-given link: 

https://docs.microsoft.com/en-us/azure/synapse-analytics/spark/apache-spark-performance-hyperspace?pivots=programming-language-csharp