When a new VLAN is created, it typically exists on a different subnet. If DHCP servers are on a different VLAN, the network needs an IP helper address to forward DHCP requests correctly. Without it, clients in the new VLAN won’t receive an IP address.

Breakdown of Options:

A. Scope size – Increasing the DHCP scope would not resolve the issue if requests aren't reaching the server.

B. Address reservations – Reservations only assign specific addresses to devices; they do not fix DHCP communication issues.

C. Lease time – Changing the lease time does not impact DHCP functionality across VLANs.

D. IP helper – Correct answer. This forwards DHCP requests across VLANs to the DHCP server.