To create three subnets with at least 30 hosts each:
Subnet Calculation: Each subnet must support 32 addresses (30 usable + 2 reserved for network and broadcast).
Bits for Subnets: Subnet mask 255.255.255.192 (/26) provides 64 addresses per subnet (4 subnets total).
Valid Subnets: Each subnet provides sufficient IPs to meet the requirement.
255.255.255.128 (A): Creates only 2 subnets, not enough for the requirement.
255.255.255.224 (C): Provides 8 subnets but only 30 addresses per subnet (28 usable), insufficient for hosts.
255.255.255.240 (D): Too restrictive, only 16 addresses per subnet (14 usable).