ANS:D - DOS=UMB

Regarding computers, a terminate-and-stay-resident program (commonly referred to by the initialism TSR) is a computer program that uses a system call in DOS to return control of the computer to the operating system, as though the program has quit, but stays resident in computer memory so it can be reactivated by a hardware or software interrupt. [1] This technique partially overcame DOS's limitation of executing only one program, or task, at a time. TSR is unique to DOS and not used in Windows.

Some terminate-and-stay-resident programs are utility programs that a computer user might call up several times a day, while working in another program, using a hotkey.

UMB (Upper Memory Blocks):

Other TSRs serve as device drivers for hardware that the operating system does not directly support.