Answer B: Asymmetric cryptography, because the receiver has its own decryption key and cannot create encrypted messages, because he doesn't have the encryption key.
About the other answers:
A: Private keys are always involved, there has to be some secret to do encryption, so this answer doesn't say very much.
C: Digital signatures can be used to verify/prove some data comes from some person (the signer) in an unaltered way. It is not about encryption, so is not about the security of your message. You use it in combination with encryption.
D: Like A, this doesn't say very much. Shared secret keys (symmetric keys) can be used for encryption, but they are less secure since the keys have to exist in more than one place (sender and receiver).
Answer:
8 (eight)
Explanation:
A piano keyboard is a musical instrument, with tabs of white and black keys that makes sound ranging from a low pitch to a high pitch sound. Its keys a labelled from A to G and occur several times on the keyboard.
If a letter "C" is pressed on the keyboard, a count of eight keys gives another occurrence of "C" again, whether it is counted upwards or downwards , but it gives a slightly higher pitch than the former "C".
The answer is HTTP and it works over the application layer. The hypertext transfer protocol or as called as HTTP is an application protocol for distributed, collaborative and hypermedia information systems in which this is the foundation of the data communication for the world wide web or as called as www.