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Correct answer: D. Extended thinking, which suits complex reasoning with no recent data needed
Explanation
The principle — pick the capability by asking what the problem is short of. If nothing external is missing, the answer is reasoning rather than retrieval.
Why the key is correct — Anthropic describes extended thinking as suited to complex reasoning tasks that do not require recent information from the web, naming mathematical problems, debugging code and analysing philosophical concepts, and allowing Claude to explore several angles before concluding. A logic puzzle with all its information present is that shape exactly.
Why the others are wrong — web search is documented for straightforward factual queries answered in one or two calls, and there is no external fact missing. Research targets comprehensive gathering across many sources over minutes, which a self-contained puzzle does not need. And multi-step reasoning is well within scope.
Remember this — missing facts call for search; missing reasoning calls for thinking.
Sources — Anthropic's guidance on when to use each capability.
Sources
“Extended thinking shines when tackling complex reasoning tasks that don't require recent info from the web—like solving mathematical problems, debugging code, or analyzing philosophical concepts.”
“Web search is best for straightforward, factual queries that can be answered with one or two tool calls”
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