Guitar strings often go "dead"--become less responsive and bright in tone--after a few weeks of intense use. A researcher whose son is a classical guitarist hypothesized that dirt and oil, rather than changes in the material properties of the string, were responsible.

option (E) Determining whether smearing various substances on new guitar strings causes them to go dead. Of the various substance he will use some will be oily and greasy. The +ve or -ve result will help him to conclude whether dirt and oil make the guitar dead.

Yes E will be the answer.

The researcher tells us that dirt and oil rather than material properties of the string are responsible for strings to go dead and ques asks which investigation will evaluate the researcher hypothesis. and therefore if we apply various substances on the string and see the result then we will see what the substance working on the string.

Guitar strings often go "dead"--become less responsive and bright in tone--after a few weeks of intense use. A researcher whose son is a classical guitarist hypothesized that dirt and oil, rather than changes in the material properties of the string, were responsible.

Effect : Guitar Strings go dead after few weeks of intense use
Cause: Dirt and oil and not changes in material properties.

What do we need to do help the classical guitarist's son? Prove that either Dirt and Oil cause guitar strings to go dead or prove that changes in material properties don't make guitar strings to go dead..

Lets attack the options.

Which of the following investigations is most likely to yield significant information that would help to evaluate the researcher's hypothesis?

(A) Determining if a metal alloy is used to make the strings used by classical guitarists - Ok, but we can't prove what causes the strings to go dead. OUT
(B) Determining whether classical guitarists make their strings go dead faster than do folk guitarists - Out of scope
(C) Determining whether identical lengths of string, of the same gauge, go dead at different rates when strung on various brands of guitars - rates at which strings go dead is not the question, OUT
(D) Determining whether a dead string and a new string produce different qualities of sound - Qualities of sound..out of scope
(E) Determining whether smearing various substances on new guitar strings causes them to go dead - If smearing different substances on guitar strings cause them to go dead then we can prove that Dirt and Oil can cause guitar strings to go dead. Its not as direct as "Determining whether smearing dirt and oil on new guitar strings causes them to go dead" but its close.

E it is by POE.