Option B is retained because the subnetting task depends on the exhibit options and the chosen plan must divide 10.1.8.0/21 into nonoverlapping partner VPN subnets without allocating more address space than required. The design principle is variable-length subnet masking: allocate the smallest subnet that satisfies each partner requirement, then place the subnets on valid prefix boundaries inside the reserved aggregate.
Cisco addressing design emphasizes conserving address space, avoiding overlap, and preserving summarization where possible. A /21 provides 2048 total addresses, so the designer can carve several smaller partner ranges while keeping the entire allocation summarizable as 10.1.8.0/21 toward the data-center core.
Incorrect options would either waste addresses, overlap existing ranges, place subnets on invalid boundaries, or allocate blocks larger than necessary. Option B is the mapping that satisfies the exhibit constraints while keeping the design clean for routing, firewall policy, and VPN onboarding. Reference topics: IPv4 VLSM, subnet boundary alignment, partner VPN addressing, summarization, nonoverlapping address design.