Explanation:

A compiler is a special program translating source code from a high-level language (e.g., C++, Java) into machine code (binary instructions) that a computer's processor can directly execute. This translation happens before execution. Option (a) describes an assembler, for assembly language. Option (b) describes a program *already* in machine language. Option (d) describes a transpiler, translating between high-level languages, not to machine code. Thus, 'A program which translates a high level language into machine language' is the most accurate definition of a compiler's primary function.