Which program is what results when assembly language programs are converted to machine language using an assembler?

The result of assembling an assembly language source is an executable. The name of that executable is dependent upon the output file name passed to the assembler. The program that performs the assembly is the assembler itself.

More specifically, the assembler produces one or more object files, which are fed into the linker or binder. The linker or binder then produces the executable, by combining the object files, along with referenced library files, and then resolving external references.