Here's a very distilled summary of the argument, which highlights the link assumption at its core:

Premise: In [this very common circumstance], traditional journalistic ethics is not adequate.
[Necessary assumption]
—
Conclusion: For most ethical dilemmas, traditional journalistic ethics is not adequate.

Per the stem, we're trying to disprove Marcus' claim about "most ethical dilemmas." For Anita's rebuttal to be relevant, the counterexample she brings up must involve an ethical dilemma.

The summary above might seem unrealistic in a timed setting. Like okay Mike, you had 3 hours to write an explanation with full hindsight, but how would I actually get this question right?

But distilling Marcus' and Anita's statements enough to reveal the link assumption is doable. Here's how.

First off, the question's stem is unique and highly informative – it tells us we're in a necessary assumption question, the structure of which will look like this:

Premise: [Anita's premises]
[Necessary assumption]
—
Conclusion: [Negated version of Marcus' claim about traditional journalistic ethics]

Before we even reach the stimulus, we know to look out for [Marcus' general claim] and negate it. That's our conclusion, which, in a necessary assumption question, is HUGE:

Conclusion: It is not true that [for most ethical dilemmas the journalist is likely to face, traditional journalistic ethics is clear, adequate, and essentially correct].

As in every other NA question, understanding the conclusion's scope is critical. This is a claim about most ethical dilemmas the journalist is likely to face.

We support our conclusion using Anita's counterexample: here's a very common situation for which this guidance is not adequate. If we ignore the link assumption, the argument is air tight:

Premise: In the typical [case], traditional ethics is not adequate.
—
Conclusion: It is not true that in most [cases], traditional ethics is adequate.

All to say, you have plenty of reason up front to be laser-focused on the "ethical dilemmas" piece of this puzzle, since it defines the conclusion's scope. You then have plenty of reason to pressure-test Anita's counterexample against the precise conclusion. If you don't notice the gap between Anita's "typical case" and ethical dilemmas at first, the argument should look valid, which gives you reason to double-back and interrogate it further.