The correct answer is: B. bytes
A byte is a unit of digital information that most commonly consists of eight bits. A bit is the smallest increment of data on a computer. It is a binary digit, meaning it can hold one of two values: 0 or 1. Bytes are used to represent numbers, letters, and other characters.
In pub/sub, a message is an opaque blob of bytes. This means that the contents of the message are not visible to the subscriber. The subscriber only knows that the message has been received and that it is a certain number of bytes long.
The other options are incorrect because they are not units of digital information. A bit is the smallest increment of data on a computer, a word is a group of bits, and a nibble is half a byte.