Assembler is the correct answer.

An assembler is a software program that translates the entire user-written, high-level language program to object code at once. Let's understand this in detail:

1. High-level language program:
- High-level language programs are written by programmers using languages like C, C++, Java, Python, etc.
- These languages are designed to be easily understood by humans and are closer to natural language.

2. Object code:
- Object code is a low-level representation of the program that can be directly executed by the computer.
- It consists of machine instructions and data that the computer can understand and execute.

Now, let's discuss the different options mentioned in the question to understand why the correct answer is "assembler":

a) Operating system:
- The operating system is responsible for managing computer resources and providing services to user programs.
- It does not directly translate high-level language programs to object code.

b) Compiler:
- A compiler is a software program that translates high-level language programs to object code.
- However, compilers typically translate programs in a source file one line or one function at a time, rather than the entire program at once.

c) Interpreter:
- An interpreter is a software program that translates and executes high-level language programs line by line.
- It does not produce object code as output.

d) Assembler:
- An assembler is a software program that takes the entire high-level language program as input and translates it directly to object code.
- It is specifically designed to handle the entire program at once.

In summary, the correct answer is "d) Assembler" because it is the software program that translates the entire user-written, high-level language program to object code at once.