Which of the following describes a characteristic of a strong encryption key?

A strong encryption key is defined by its long length, randomness, and complexity.

The length of an encryption key is crucial because longer keys can provide exponentially greater security against brute-force attacks, making it significantly more difficult for unauthorized parties to decrypt data. A key that is random ensures that it is less predictable, which minimizes the risk of an attacker guessing or deducing the key based on patterns or common sequences. Complexity further adds to the key's strength by incorporating a mix of characters, numbers, and symbols, which complicates potential attacks since patterns are not easily recognizable.

The other options do not contribute to the effectiveness of an encryption key. For example, short keys or those that are simple lack the necessary security to withstand modern-day cryptographic attacks. Fixed-length keys that follow predictable patterns are easier to crack, and keys that are easy to remember and commonly used are often vulnerable to guesswork and dictionary attacks. Thus, the combination of long length, randomness, and complexity makes option B the defining characteristic of a strong encryption key.