Overview:
The question asks which of the given options is generally costlier. The options include a server, notebook computer, personal computer, and mainframe. The correct answer is option 'D' - mainframe. This response will provide an explanation of why mainframes are generally costlier compared to the other options.

Explanation:
Mainframes are high-performance computers that are designed to handle large-scale computing and data processing tasks. They are typically used by large organizations and enterprises for critical applications and massive data processing requirements. Here are the reasons why mainframes are generally costlier compared to the other options:

1. Complexity and Specialization:
- Mainframes are highly specialized machines designed for specific tasks, such as transaction processing, data analytics, and large-scale storage. They are built with advanced technologies and components to handle high workloads.
- The complexity involved in designing and manufacturing mainframes results in higher production costs compared to other computer systems.

2. Scalability and Capacity:
- Mainframes have exceptional scalability and capacity. They can handle massive amounts of data and accommodate a large number of concurrent users.
- The ability to scale and handle enormous workloads requires significant investment in hardware, including processors, memory, storage, and networking components, which adds to the cost.

3. Reliability and Availability:
- Mainframes are known for their high reliability and availability. They are designed with redundant components, fault-tolerant architecture, and advanced error detection and correction mechanisms.
- Building and maintaining such high levels of reliability and availability involve additional costs, including redundant hardware, advanced cooling systems, and continuous monitoring and maintenance.

4. Maintenance and Support:
- Mainframes require specialized maintenance and support services. They often come with dedicated support teams from the manufacturers or third-party vendors.
- The cost of maintenance contracts, regular updates, and support services adds to the overall cost of owning and operating a mainframe system.

Conclusion:
In conclusion, mainframes are generally costlier compared to servers, notebook computers, and personal computers due to their complexity, specialization, scalability, reliability, availability, and the need for specialized maintenance and support services.