Prob $3\mathrm{.}18$

a$.$

This method is not secure$.$

b$.$

If Alice encrypts plaintext one letter at a time, then the same letter will have the same ciphertext. The attacker can figure out letter frequencies to decrypt the cipertext.

c$.$

The Hamming distence between every two adjacent letters is $1,$ which makes it easy for attacker to carry out cryptoanalysis.

d$.$

None of the above.

e$.$

All a$,$ b$,$ and c$.$