What does the key length represent

A.
Hash block size

B.
Cipher block size

C.
Number of permutations

D.
???

Explanation:
BD
In cryptography, an algorithm’s key space refers to the set of all possible permutations of a keys.
If a key were eight bits (one byte) long, the keyspace would consist of 28 or 256 possible keys. Advanced
Encryption Standard (AES) can use a symmetric key of 256 bits, resulting in a key space containing 2256 (or
1.1579 × 1077) possible keys.
Source: https://en.wikipedia.org/wiki/Key_space_(cryptography)