Although Peter Harmann does make very good points in his answer and explains everything perfectly, I think the only correct answer can be #4.
Before the block encrypt you can only get identical ciphertext blocks if the input of the block cipher is identical in both CBC mode encryption operations. Block ciphers are permutations so there is always a 1:1 relation between a specific plaintext block and a specific ciphertext block.
The problem is that the question doesn't make it explicit that the question is about the block cipher rather than the block cipher in CBC mode. The ciphertext block directly before the block encrypt is XOR'ed with the plaintext block before the block encryption is performed. So the quality of the question is certainly up for discussion.
To make sure that identical ciphertext blocks aren't found there needs to be a maximum number of blocks that can be encrypted by CBC mode. That maximum is related to the block size of the block cipher being used. This is one of the reasons why AES (128 bit block size) should be preferred over triple DES mode or blowfish (64 bit block size).
If an identical block is found then the XOR between the two plaintext blocks is leaked, and leakage of any kind of data can be abused and breaks a cipher.
There may be a (final) move to 256 bit block ciphers in the future because of issues like this. Common/modern modes of operation such as CTR (counter mode) and GCM have limitations on the amount of message bytes and number of messages that can be encrypted with a single key, despite operating on a 128 bit block size.