A program that is used by other routines to accomplish a particular task is called:

Explanation

A subroutine is, by definition, a self-contained block of code written to perform one specific task and designed to be called (invoked) by other parts of a program whenever that task is needed, which exactly matches the question's description. A microprogram is the lowest-level sequence of micro-instructions implementing a CPU instruction, and a micro-operation is a single register-transfer-level step within one instruction cycle — both belong to the hardware/control-unit level, not to being 'called by other routines'. A plain 'routine' is too generic a term and is not the standard name for a sub-task called by other code. Hence option (iv) Subroutine is correct.