Explanation:

A **turnkey system** is a type of computer system that is sold to a customer as a complete package, ready to use immediately upon delivery and installation. The term 'turnkey' implies that the user simply needs to 'turn a key' to start using it, without needing to integrate different components or install extensive software. This means it includes both the necessary **hardware** (physical components) and the required **software** (programs and operating system) to perform its intended function. Option (b) 'Complete software' and (d) 'Complete hardware' are incomplete descriptions, as a true turnkey system encompasses both. Option (c) 'Complete language' is irrelevant in this context. Hence, option (a) provides the most accurate and comprehensive definition of a turnkey system.