Answer:
Usually, it's because of taxes and insurance.
Skype isn't a cloud based system. Cloud based systems are online storage systems. Google Drive, Dropbox, and SkyDrive are all pieces of software which allow you to store data on a remote server over the internet. Skype is just a program.
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:
Watermark
Explanation:
This is actually called a Watermark. It is usually used as a way of showing the name or logo of the author who owns the content that is underneath the watermark. This is done so that individuals cannot steal the owner's work and use it as their own. Usually, the owner has the original version of the content without any watermark, as well as individuals that have purchased the content or a licence to use the content for personal or commercial usage.
