DES is also known as a Symmetric Key or Secret Key algorithm. DES is a Symmetric Key algorithm, meaning the same key is used for encryption and decryption.
For the exam remember that: DES key Sequence is 8 Bytes or 64 bits (8 x 8 = 64 bits) DES has an Effective key length of only 56 Bits. 8 of the Bits are used for parity purpose only. DES has a total key length of 64 Bits.
The following answers are incorrect:
Two-key This is incorrect because DES uses the same key for encryption and decryption.
Asymmetric Key This is incorrect because DES is a Symmetric Key algorithm using the same key for encryption and decryption and an Asymmetric Key algorithm uses both a Public Key and a Private Key.
Public Key. This is incorrect because Public Key or algorithm Asymmetric Key does not use the same key is used for encryption and decryption.
References used for this question: http://en.wikipedia.org/wiki/Data_Encryption_Standard