$5.$ DHCP:

$($a$)$ allows an ISP to obtain a set of IP addresses from the ICANN $($Internet Corporation for Assigned Names and Numbers$).($b$)$ allows a router to allocate port numbers in a NAT $($Network Address Translation$).($c$)$ allows a host to dynamically obtain an IP address when it joins the network. $($d$)$ both $($a$)$ and $($c$).$