Final answer:
The effective key size of DES is 56 bits, which is reflected in option A). DES uses a 64-bit key with 8 bits reserved for parity, leaving an effective 56 bits for encryption.
Step-by-step explanation:
The effective key size of DES (Data Encryption Standard) is 56 bits. DES was originally designed with a 64-bit key, but only 56 bits are actually used for encryption. The remaining 8 bits were used for parity checking, making them non-functional in terms of contributing to security. Therefore, the correct option for DES's effective key size is A) 56 bits.
Although DES was considered secure for many years, the 56-bit key size eventually became vulnerable to brute-force attacks. This prompted the development of more secure encryption standards with longer key lengths, such as Triple DES and AES, which have key sizes of up to 256 bits, greatly enhancing security.