(A) tells us the list was shorter than the paragraph because some words appeared multiple times in the paragraph but only once in the list. This is a fact about the two tasks, but it applies equally to both groups of readers. It doesn't explain why beginning readers always performed better on the paragraph while experienced readers performed better on whichever task came second.

This tells us experienced readers were better at using context than beginning readers. But if experienced readers had a bigger advantage from context, we'd expect them to always do better on the paragraph (since the paragraph is the task that provides context). Instead, experienced readers did better on whichever task came second, even when that task was the list of randomly ordered words with no context at all. So (B) doesn't fit the experienced readers' pattern.

Beginning readers relied solely on context to guess at difficult words. That means context is the only tool in their toolkit. When they read the paragraph, they have context to lean on, so they make fewer errors. When they read the list, the words are randomly ordered, so there's no context to use, and they struggle. And, even if they read the paragraph first and encountered all the words already, that prior exposure doesn't help them with the list, because they don't sound out words. Their only method is context, and context is either present (paragraph) or absent (list). That's why order of the tasks doesn't matter for them.

Like (A), this is a fact about the tasks that applies equally to both groups. Yes, both tasks use the same words, so children would recognize words in the second task. But this is exactly the fact that makes the beginning readers' pattern puzzling in the first place. If seeing the same words again should help (and it does help the experienced readers), why doesn't it help the beginning readers?

This tells us beginning readers made more errors overall than experienced readers on both tasks. That's not surprising, and it doesn't address the question. We're not asking why beginning readers make more mistakes in general. We're asking why the order of the tasks didn't affect beginning readers' performance. The fact that they made more total errors doesn't explain why they always did better on the paragraph regardless of whether it came first or second.