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).
