What happens if the key is shorter than the plaintext in a Vigen$$re cipher?

Question $13$Answer

a$.$

The key is repeated to match the length of the plaintext.

b$.$

The encryption cannot be performed.

c$.$

None of these

d$.$

The key is truncated to match the length of the plaintext.

e$.$

The plaintext is shortened to match the length of the key.