What translates and executes source code into machine language one line at a time?

An assembler converts assembly language, which is a low-level programming language, into machine code. Unlike interpreters, assemblers translate the entire code at once rather than executing it line by line. Therefore, it is not the correct choice for the question regarding translating and executing source code one line at a time.

An operating system manages computer hardware and software resources and provides common services for computer programs. While it may facilitate the execution of programs, it does not translate source code into machine language. Its function is fundamentally different from that of an interpreter.

A compiler translates the entire source code into machine language before execution, creating an executable file. This batch-processing approach contrasts with the line-by-line execution characteristic of interpreters, making compilers unsuitable for the question posed.