(A) Eighteenth-century instruments cannot be used to provide information about the original techniques used in playing such instruments if they have been restored.
We can play an instrument after restoring it and learn about the original techniques used in playing these instruments. This is incorrect.
(B) Eighteenth-century instruments that have been restored can provide information only about how eighteenth-century music originally sounded
This statement states that the restored instrument can provide only ..... this information. The use of "only " makes this choice extreme, and thus, it is incorrect.
(C) Eighteenth-century instruments are the only source of information about the instrument-making techniques of that period.
No, there can be other ways of knowing about instrument-making techniques such as manuscripts etc.
(D) An eighteenth-century instrument that has not been restored can provide more information than can one that has been restored.
We don't know what information here is referring to. thus, This choice is not correct
(E) An eighteenth-century instrument cannot serve as a source of new information about eighteenth-century instrument-making techniques once it can be played
If the instrument is played, that means it has been restored. Thus, that instrument cannot provide new information about the 18th century instrument making. This is the correct answer choice.
Correct answer choice is E.