Explanation:

Supercomputers are the most powerful and fastest computers available, designed to perform complex calculations at extremely high speeds. They are indeed (b) 'fast and costly computer systems' due to their advanced architecture and specialized components. They are typically (c) 'used in large organisations' for highly demanding tasks such as weather forecasting, scientific research, simulations, and complex data analysis. Furthermore, they are often designed to handle (a) 'data of more than one user at a time' through parallel processing and sophisticated resource management, enabling multiple researchers or processes to utilize their immense power concurrently. Since all these statements accurately describe a supercomputer, option (d) 'All of these' is the correct answer.