Final answer:
In the context of encryption modes, Cipher Block Chaining (CBC) propagates transmission errors because each block depends on the previous one, while Output Feedback Mode (OFB) does not propagate errors beyond the block with the error.
Step-by-step explanation:
The student's question relates to modes of encryption and their ability to propagate transmission errors. Transmission errors are propagated in the mode that allows an error to affect not only the block containing the error but also subsequent blocks. For example, in Cipher Block Chaining (CBC), if a bit error occurs in transmission, it will affect the decryption of the block containing the error and all subsequent blocks until the error is corrected. This happens because in CBC each cipher block is dependent on the previous cipher block.
On the other hand, in Cipher Feedback Mode (CFB) and Output Feedback Mode (OFB), errors do not propagate beyond the affected block because future blocks do not depend on the decryption of previous ones. Therefore, the correct answer to the question 'Transmission errors are propagated in the following modes, except __________,' would be (b) Output Feedback Mode because error propagation is limited to the incorrect block itself without affecting subsequent blocks.