This is the opinion or claim which Eric is trying to persuade his audience to agree with. After declaring his claim, Eric presents thee arguments in its support. Hence this option (cell phones are academically beneficial) presents the central idea of this persuasive excerpt.
Option A is incorrect because here Eric is presenting a claim by some people which he would try to refute in next few lines.
Option C and D are incorrect because they are not ideas or claims but arguments in support of the central idea/claim.
