In $2004,$ a large department store chain in Germany was forced to withdraw customer loyalty cards when the press discovered that the company had secreted radio$-$frequency identification $($RFID$)$ tags in the plastic. Even when enclosed in a wallet or purse, these cards can transmit information over distances of about a meter, and the stores planned to use them to track which departments the customer visited. At issue was which data security issue?

A$.$ unauthorized access

B$.$ chat rooms

C$.$ biometrics

$($D$)$

D$.$ consumer privacy