To educate others about his (23) theories Einstein held some teaching positions.

Right.

Thank you

You're welcome.

I think you'll find out it's B.

ok...thank you

Actually, the correct answer is D) no error. There is no grammatical error in the given sentence.

To explain further, when we refer to someone, we usually use their last name without any punctuation marks unless the context requires it (e.g., in formal writing or when distinguishing between multiple individuals with the same last name). In this sentence, "Einstein" is used to refer to Albert Einstein, and there is no need for a comma or any other punctuation mark after it.

Thus, the sentence is grammatically correct as it is.