Ciphers. A substitution cipher is a function $E_k$:$LS$ that for each secret key $k$ and a letter from an alphabet $L$ of constant size outputs a ciphertext symbol

from a ciphertext alphabet $S$ of constant size. A string is encrypted by encrypting each letter separately using the same key $k.$

Is it possible to construct a secure substitution cipher?

a$)$ No

b$)$ Yes

c$)$ Sometimes