The best method for creating the records in Salesforce based on the requirements is:
C. REST API request using composite/vbatch/.

The REST API request using composite/vbatch/ allows you to create up to 200 records in a single request, and returns the response immediately. This method is suitable for scenarios where you need to create records quickly and do not need to synchronize data between systems.

The Bulk API 2.0 is designed for loading large batches of data asynchronously, and does not return the response immediately. This method is suitable for scenarios where you need to load or delete millions of records and do not need real-time feedback.

The Apex web services allow you to expose custom logic as a SOAP or REST web service, and require you to write Apex code to handle the requests and responses. This method is suitable for scenarios where you need to implement complex business logic or integrate with external systems that use SOAP.

The REST API request using composite/tree/ allows you to create one or more sObject trees (a collection of nested, parent-child records with a single root record) in a single request, but all tree root records must be the same type. This method is suitable for scenarios where you need to create hierarchical data structures and maintain referential integrity.

https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/resources_composite_composite.htm
https://resources.docs.salesforce.com/latest/latest/en-us/sfdc/pdf/api_rest.pdf...