Explanation:

Compiling is the process of translating source code written in a high-level programming language (like C++, Java, or Python) into machine code or an intermediate code that a computer's processor can directly understand and execute. The direct output of a successful compilation process is an 'executable program' (or an object file that can be linked to form an executable). This executable file contains the machine instructions that the CPU can run. Option (a) 'program specification' is a detailed description of what a program should do, created *before* coding. Option (b) 'algorithm' is a step-by-step procedure for solving a problem, which is the logical foundation *behind* the code. Option (d) 'subroutine' (also known as a function or procedure) is a block of code within a larger program, but compiling creates the entire runnable program, not just a subroutine in isolation as its primary output. Thus, an 'executable program' is the correct outcome of compilation.