Answer:
(B) The field contains unique record identifiers from a system outside of Salesforce.
(C) The field can be unique based on case-sensitive or caseinsensitive values.
Step-by-step explanation:
Whe you are using and external ID field that means that for example you could be using data from other software to identify an account or an ID within the software that you are using, so you´d need unique record identifiers from that software, and the field could be unique based on case-sensitive or case insensitive values, this is in order to not have duplicates within sales force, this can helpp you keep the software and data base running smoothly.