A. Only one uses a symmetric key.
B. One uses a key pair set.
C. One uses a mechanism that produces a bit string the same length as the message.
D. Only one provides digital enveloping.
The correct answer is C.
Domain: 6.1. In relation to a block cipher, the key length and the block length are not linked or related; they are simply defined/prescribed by the algorithm. In relation to a stream cipher, the key is used to engage a key stream generator that produces a key bit stream that is the same length as the message. Both are forms of symmetric encryption — neither uses a key pair set nor is used for digital enveloping.