Which of following is a characteristic of electronic code book $($ECB$)$ mode?

a$.$ Only one character is processed at a time.

b$.$ It requires access to a synchronous counter for both the sender and receiver of the message.

c$.$ Each block of plaintext is XORed with the previous block of ciphertext before being encrypted, making it susceptible to attacks.

d$.$ Two identical plaintext blocks are encrypted into two identical ciphertext blocks, making them susceptible to attacks.