...select the instruction set for a single C or C++ function?

I have a C/C++ source file that is compiled in Thumb mode, however I want one of the functions to be compiled in ARM mode. Is it possible to set the instruction set of a single function?

No, the instruction set can only be set for the entire source file using the Compiler > Instruction Set project property. You will need to put the function into another source file and set the instruction set property of this source file to the required value.