When I compile your program, I get the following linker errors:
1>main.obj : error LNK2019: unresolved external symbol "double __cdecl surfaceAreaCylinder(double)" (?surfaceAreaCylinder@@YANN@Z) referenced in function _main
1>main.obj : error LNK2019: unresolved external symbol "double __cdecl volumeCylinder(double)" (?volumeCylinder@@YANN@Z) referenced in function _main
This indicates that your function protypes do not match your implementation.
You declare surfaceAreaCylinder as follows: double surfaceAreaCylinder (double nSurfaceAreaCylinder);
But the implementation is: