First, let me explain the options that I rejected as Answer
1 - External Id: - External Id can be Text, Email, Number so it cannot refer the record Id if the type is Number OR Email
2 - ENUM: - we can not store the id in Enumeration
Now, coming to your question sObject is the universal object in Salesforce which contains an Object that may be Standard one or Custom one and believe you already are already familiar about this. And we can store any object in sObject we can get the Id as well.
So the ans is String and sObject
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AK2020
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