As this seems to be homework, I'll try not to just give you the answer, but to give you some questions and thoughts which should help you. Also, here are some links which should be interesting:
a)The communications require a key exchange not supported by block ciphers.
Can you exchange a key via a block cipher? For sure, you can exchange anything you want via a block cipher.
But how do you encrypt the key exchange? What key do you use and how did you exchange that?
b)The communications are not of a predictable length, and so a block cipher may have to wait for more traffic to appear.
It is true that block ciphers need input that is a multiple of its block size. But are there possibly better solutions than waiting for more input? What if there is no more input?
c)Block ciphers are too computationally expensive for this purpose.
Well, if that is the case, what would be faster ciphers? Possibly some sort of public key encryption?