"Describe the Argument" questions are nice in the sense that we're just picking our answer based on whether the answer choice matches something that happened.
The correct answer says something true about the argument; the four wrong ones say something inaccurate.
(A) is correct because Rossi does make an appeal to a general principle. The first sentence is a general principle.
There's no clear cut definition I can give you for 'principle' other than:
- it should be something of a generalization
- it should have the feel of a rule / standard
"Should" statements will sometimes be principles. Principles will sometimes be "should" statements. But we shouldn't equate the two. (That last sentence was a principle)

If I say, "Molly should really take a bath", that's not a principle because it's not generalized.
If I say, "Dirty people should really take a bath", that's a principle.
Principles don't have to use 'should' / 'ought' / 'right' / 'wrong'. They can just imply a standard to be followed.
Many famous cliches that essentially sound like advice for living could be called principles, for example
"If it ain't broke, don't fix it."
"Dress for the job you want, not the job you have."
etc.
The conclusion here, "children should have the right to vote", could fairly be called a principle. But
making an appeal to a principle would have to be describing a premise, not the conclusion.
In trying to convince us that children should have the right to vote, Rossi appeals to the principle that people living in a democracy should have their interests represented.
So (A) is correct because it describes the 1st sentence.
=== other answers ===
(B) Rossi doesn't address any opponent, so he definitely doesn't "deny the good faith" of an opponent.
(C) The only proposal Rossi has is that 'children should have the right to vote'. Does Rossi 'evaluate the predictable consequences' of what would happen if children got the right to vote? No. Eliminate.
(D) This accuses Rossi of offering no rational for a policy. The policy Rossi is discussing is 'giving children the right to vote'. He
does offer rationale: it's undemocratic to not have your interests represented by the govt. and children sometimes have different interests from their parents.
(E) Rossi uses the term 'interest' twice, but he uses it in the same sense both times.
Hope this helps. Let me know if questions remain.