What is defined as a group of bits representing a complete piece of information?

The correct choice is a byte, as it is a standard unit of digital information that typically consists of eight bits. A byte can represent a single character of data, such as a letter or number, due to its capability of being in 256 different states (from 0 to 255 in decimal notation). This property makes it an integral part of data storage and processing in computer systems.

While bits, which are the smallest units of data, represent a binary state (either 0 or 1), they by themselves do not complete information; they require grouping to provide meaningful data. A "word" technically refers to the natural data size that the CPU processes at one time, and it can vary in size depending on the architecture (such as 32-bit or 64-bit), but it is not as universally defined for representing a single piece of information as a byte is. A "nibble" is half a byte, consisting of four bits, and is also insufficient on its own to represent a full character or a complete piece of information. Therefore, the byte is the most appropriate and widely accepted definition in this context.