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Einstein………………is famous for his theory of relativity.

a) who was born in Germany

b) , who was born in Germany ,

I go for 'b' because this is a non-defining relative clause. But is 'a' completely wrong?

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Omar AhmedI go for 'b' because this is a non-defining relative clause.

Correct. Relative clauses after a proper name are always non-defining. The name itself defines the referent. It doesn't need anything else to narrow down the meaning of the name. But see the notes below.

Omar AhmedBut is 'a' completely wrong?

In this sentence, yes. However, if you are trying to differentiate between two or more people with the same name (Einstein), you can write it like this:

The Einstein who was born in Germany is famous for his skill at making sausage. (In other words, not the other Einstein we were also talking about, possibly the Einstein who was born in the United States.)

CJ