It is now a common complaint that the electronic media have corroded the intellectual skills required and fostered by the literary media.
Conclusion (so)
Electronic media is probably just altering the human mind, not devolving it.
Evidence
Several centuries ago people complained that the spread of literacy was destroying certain intellectual skills of the oral culture.
Evaluate
When read arguments on Role (and Main Conclusion) we know about half of them are rebuttal style. So if the paragraph begins with an Attribution, such as "it is now a common complaint that X", then we'll anticipate a Pivot from our author into a rebuttal.
Sure enough, the second sentence begins with the Pivot 'but'. Pivots almost always mean that we're leaving the counterpoint / background area of the stimulus and entering the Author's Argument. The fact that the structure is, "But claim 1. So, claim 2" tells us that claim 1 is support and claim 2 is the conclusion.
Rebuttal arguments are usually refuting the original claim in the Attribution. In this case, is our author arguing that, "electronic media are NOT corroding our literary skills?"
It's kinda hard to say. She might be arguing,
Or she might be saying, as a way to to say that, . It's just ALTERING us, not making us worse (devolving us).
Goal
We're being asked about the second sentence. We identified that as the Premise, which is supporting the conclusion in the third sentence.