The correct answer is: B. EMM386.exe
EMM386.exe is a DOS extender that allows programs to access expanded memory. It does this by creating a virtual machine that runs in the background and provides access to expanded memory.
Himem.sys is a DOS driver that allows programs to access extended memory. It does this by mapping extended memory to conventional memory.
Mem386.sys is a DOS driver that provides memory management services. It does not emulate expanded memory.
Ramdrive.sys is a DOS driver that creates a RAM disk. It does not emulate expanded memory.
Expanded memory is a type of memory that was used in early PCs. It was more than the 640KB of conventional memory that was available to DOS programs, but it was not as fast as conventional memory. EMM386.exe allowed programs to access expanded memory, which made them run faster.