a. modem or d. voice input. Not a port because a port allows one to insert a peripheral device but not really communicate. I have no idea what an expansion slot is. It could be d. voice input because this allows a person to hold an audible conversation.
But I'd go for modem because a modem lets you connect devices like laptops, tablets, phones, computers etc. and share data between them. So things like mailing are possible (I THINK)!
