Final answer:
Carmustine (BCNU) is the drug wafer that may be placed directly into the cavity created during surgical brain tumor removal.
Step-by-step explanation:
The drug wafer that may be placed directly into the cavity created during surgical brain tumor removal is carmustine (BCNU). Carmustine is an alkylating agent and is commonly used in the treatment of brain tumors. It is delivered directly to the tumor site to target and eliminate any remaining cancer cells.