Routing in production planning and control refers to the sequence of operations to be performed in order to complete a particular task or project. It involves determining the most efficient path for the flow of materials, information, and resources throughout the production process. Here is a detailed explanation of why option 'D' is the correct answer:

1. **Definition of Routing**: Routing is the process of determining the most effective and efficient sequence of operations that need to be performed to complete a specific task or project. It involves identifying the steps, work centers, and resources required for each operation.

2. **Sequence of Operations**: One of the key aspects of routing is determining the correct sequence of operations. This involves analyzing the requirements of the task or project and identifying the logical order in which the operations need to be performed. By establishing the correct sequence, the production process can be optimized for efficiency and productivity.

3. **Optimization of Flow**: Routing helps optimize the flow of materials, information, and resources throughout the production process. By identifying the most efficient path, unnecessary delays, bottlenecks, and inefficiencies can be minimized or eliminated. This ensures smooth and uninterrupted flow, reducing lead times and improving overall productivity.

4. **Resource Allocation**: Routing also plays a crucial role in balancing the workload on machines and work centers. By determining the sequence of operations, it becomes easier to allocate resources effectively. This helps prevent overloading or underutilization of machines, ensuring optimal resource utilization and avoiding production delays.

5. **Scheduling and Planning**: Routing is closely related to scheduling and planning activities in production planning and control. Once the sequence of operations is determined, it becomes easier to create a detailed production schedule, allocate resources, and plan for material requirements. This helps in coordinating activities, setting realistic timelines, and ensuring that the production process operates smoothly.

In conclusion, routing in production planning and control refers to the sequence of operations to be performed in order to complete a specific task or project. It involves determining the most efficient path for the flow of materials, information, and resources. By establishing the correct sequence, routing helps optimize the production process, balance workload, allocate resources effectively, and facilitate scheduling and planning activities. Therefore, option 'D', i.e., sequence of operations to be performed, is the correct answer.