Explanation:

Unix is renowned for several key characteristics that made it a powerful and versatile operating system. One of its most fundamental features is its **multi-user capability**, meaning that **at a time more than one people can work** on the system simultaneously, each with their own processes and resources. This is directly addressed by option (b). Another core characteristic is **multitasking**, which means **at a time more than one work** (processes) can be executed concurrently, as stated in option (a). Both (a) and (b) are valid characteristics. However, if only one option can be chosen, the multi-user aspect (b) was particularly significant for Unix's adoption in server environments and shared computing. While Unix is generally considered robust and secure (hence 'More safe' in option c is partially true, but subjective), and its kernel indeed manages system resources including data (option d), multi-user and multitasking are more defining high-level characteristics. Given the options, (b) highlights a distinct and important feature of Unix.