Choice "A" is correct. A digital signature is a means of ensuring that the sender of a message is authentic. The digital signature uses encryption so that the recipient of a message can be assured that it is from the sender that is shown.
Choice "d" is incorrect. A digital signature will not keep a message from being intercepted in route, any more than a normal signature would.
Choice "c" is incorrect. A digital signature will not ensure that a message is received by the intended recipient, any more than a normal signature would.
Choice "b" is incorrect. A digital signature will not ensure that a message is sent to the correct address, any more than a normal signature would.