Final answer:
Among the options, RC4 is recognized as an insecure cipher, although it is actually a stream cipher, not a block. MD5 is an insecure hash function, not a cipher. AES and ECB are considered secure, though ECB has limitations.
Step-by-step explanation:
An insecure block cipher is one that has known vulnerabilities and can be compromised with relative ease by modern standards. Among the options given, B) RC4 stands out as an insecure cipher, but it is important to note that RC4 is actually a stream cipher, not a block cipher. However, because RC4 has been widely recognized as insecure due to vulnerabilities that allow for attacks, it might be considered the correct answer if we relax the criteria to any type of cipher. Furthermore, D) MD5 is not a cipher at all; it is a cryptographic hash function, which has also been found to be insecure due to vulnerabilities to collision attacks. The other options, A) AES (Advanced Encryption Standard) and C) EBC (presumably a typo for ECB, Electronic Codebook mode if in the context of cipher modes) are considered secure; AES is a widely used and secure block cipher, whereas ECB is a block cipher mode of operation that is not secure when used without padding or with data susceptible to pattern recognition.