$(2)$ In an LTE network architecture, the function of the Mobility Management Entity $($MME$)$ is:

$($a$)$ QoS decisions

$($b$)$ Device registration and tracking, and bearer management

$($c$)$ Subscription data and security information

$($d$)$ Default gateway, packet filtering and QoS enforcement

$($e$)$ Packet data routing and mobility $$anchor$$

$($f$)$ None of the above