Which statement is true concerning digital signature?
- can be used as a cryptosystem and signature system.
No. A cryptosystem is generally a system where confidentiality, integrity and authenticity can be achieved.
So a cryptosystem will likely also require things like encryption to obtain confidentiality. And encryption is not available if you just have a signature generation & verification primitive.
Note that signature generation / verification itself is generally a hybrid cryptosystem of a symmetric hash and an asymmetrical computational hard problem such as RSA. But that's something different.
- a way of encrypting confidential data
Certainly not. Operations with a private key of the sender cannot be used to obtain confidentiality. For that you need the public key of the receiver.
- a method of sending handwritten signature to digital media
- allows the receiver of data to prove the source and integrity
Yes, the receiver can verify that the data was signed by the private key of the sender if the receiver has obtained and trusts the public key of the key pair.