Final answer:
Disabling set net-devices in config vpn ipsec phase1-interface usually results in the failure of VPN tunnel establishment as specific network devices will not be used for the IPSec connection.
Step-by-step explanation:
If set net-devices is disabled in config vpn ipsec phase1-interface, it typically means that specific network devices will not be bound to the VPN connection. Disabling this option could lead to a scenario where the VPN tunnel establishment fails (option a). However, the exact behavior may depend on the vendor-specific implementation of the IPSec protocol. Without binding to specific network devices, the VPN falls back to using the default pathways for IPSec negotiations and does not ensure traffic routes through the intended devices.