The correct answer is “56”. This is actually a bit of a trick question, since the actual key length is 64 bits. However, every eighth bit is ignored because it is used for parity. This makes the “effective length of the key” that the question actually asks for 56 bits.
The other answers are not correct because:
168 -This is the number of effective bits in Triple DES (56 times 3). 128 -Many encryption algorithms use 128 bit key, but not DES. Note that you may see 128 bit encryption referred to as “military strength encryption” because many military systems use key of this length. 64 -This is the actual length of a DES encryption key, but not the “effective length” of the DES key.
Reference: Official ISC2 Guide page: 238 All in One Third Edition page: 622