External data store helps in searching the information in other types of data stores. Here in the given scenario, the primary index of the file is file path but you want to search data on the basis of the contents of the file. So, External Index Data Store will create an external index i.e. secondary index for the data store and perform the search.
Option A is incorrect. Document data store does not provide the ability to search any information that is present in any other data store.
Option B is incorrect. Key/Value data store does not provide the ability to search any information that is present in any other data store.
Option C is correct. External Index data store does not provide the ability to search for any information that is present in any other data store.
Option D is incorrect. Graph data store does not provide the ability to search any information that is present in any other data store.
Reference: To know more about the different data stores, refer to the link below:
https://docs.microsoft.com/en-us/azure/architecture/data-guide/big-data/non-relational-data