A public key encryption system
In a public-key encryption system, any person can encrypt a message using the public key of the receiver, but such a message can be decrypted only with the receiver's private key.A public key encryption allows only the correct receiver to decode the data because others dont have the private keys.so correct answer is C
In a public-key encryption system,Basically any person can do the task of encryption using public key of receiver .
When receiver receive its cipher text ,then only he has private therefore only he can decrypt the CT and convert into PT(Plain text).
Therefore sender can be any one please but receiver will be one that is receiver.
Therefore best option will C.
The answer to the first probability question ...