Final answer:
Historical documents have the least direct evidence for climate change.
Step-by-step explanation:
The statement with the least direct evidence for climate change is d. Historical documents.
Climatologists look for climate change in many components of the climate system. While historical documents can provide some context and insights into past climate conditions, they do not directly measure climatic variables like temperature and precipitation. Instead, historical documents offer indirect evidence that must be interpreted and analyzed alongside other sources of information.