Why does key clustering represent the significant failure of an algorithm? $($D$5,$L$3\mathrm{.}5)$

A single key should not generate different ciphertext from the same plaintext, using the same cipher algorithm.

Two different keys should not generate the same ciphertext from the same plaintext, using the same cipher algorithm.

Two different keys should not generate different ciphertext from the same plaintext, using the same cipher algorithm.

A single key should not generate the same ciphertext from the same plaintext, using the same cipher algorithm.