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.
Reference:
CompTIA Network+ (N10-009) Official Study Guide - Domain 2.4: Explain IP addressing technologies and subnetting.
RFC 1542: BOOTP (Bootstrap Protocol) relay agents