The operating system plays a crucial role in managing processes in a multi-programming environment. It is responsible for making decisions regarding process execution, and one of the key decisions it makes is how much time each process gets the CPU. This is commonly known as CPU scheduling.

CPU scheduling is essential because the CPU can only execute one process at a time. In a multi-programming environment, there are multiple processes competing for CPU time, and the operating system must ensure fair and efficient allocation of the CPU to these processes. The decision of how much time each process gets the CPU is made by the operating system using various scheduling algorithms.

Here are the reasons why option 'C' is the correct answer:

1. CPU Utilization:
- The operating system aims to maximize CPU utilization to ensure that the CPU is not idle for long periods.
- By allocating specific time slices to each process, the operating system ensures that the CPU remains busy executing processes as much as possible.

2. Fairness:
- The operating system needs to be fair in providing CPU time to all processes.
- By allocating equal time slices or using priority-based scheduling, the operating system ensures that each process gets a fair share of the CPU's resources.

3. Response Time:
- The operating system strives to provide a reasonable response time to all processes.
- By allocating shorter time slices to interactive processes or processes with high priority, the operating system ensures that these processes get quick access to the CPU, resulting in better responsiveness.

4. Throughput:
- The operating system aims to increase the overall throughput, i.e., the number of processes completed per unit of time.
- By efficiently scheduling processes and providing them with appropriate CPU time, the operating system can increase the overall throughput of the system.

In conclusion, the operating system decides how much time each process gets the CPU to ensure optimal CPU utilization, fairness, responsiveness, and throughput in a multi-programming environment. This decision is crucial for efficient process execution and overall system performance.