An interpreter translates and executes source code one line at a time. In Information Technology, interpretation is a method of program execution where each statement is read, translated into machine-level actions, and executed immediately. This allows programmers to test and debug quickly because errors are often reported at the line where execution stops. Many scripting languages and interactive environments use interpreters, supporting rapid development and flexibility. Unlike a compiler, which translates the entire program into a machine code file that can be executed later, an interpreter generally requires the source code or intermediate representation to be present at runtime. An assembler is used for assembly language and converts mnemonic instructions into machine code, but it is not typically described as executing source code one line at a time. The operating system manages system resources and provides services, but it is not the tool that translates programming language source code into machine language during execution. Therefore, the correct answer for translating and executing code line by line is interpreter.