Final answer:
Supporting documents and appendix are often included as attachments to short reports. So, the correct answer is B. Supporting documents.
Step-by-step explanation:
The types of material often included as attachments to short reports are:
Supporting documents: These are documents that are related to the report or contain additional information that is not central to understanding the report.
Appendix: This includes documents that are related to the report or contain information that can be culled but are not deemed central to understanding the report.
Conclusions and recommendations may be included as part of the main body of the report rather than as attachments.
Thus, the correct answer is B. Supporting documents.