Host isolation ensures that a device cannot communicate with other systems on the network. Isolation is typically applied through EDR/XDR tools, quarantine mechanisms, or endpoint firewalls. Security+ SY0-701 identifies host isolation as a containment technique used when:

A device is suspected of compromise

Lateral movement must be prevented

Sensitive systems must be protected

A system must be completely cut off except for administrative access

Isolation enforces an immediate block on all inbound and outbound communications, effectively making the device inaccessible to any network-based resource.

Disablement of unused services (A) reduces attack surface but does not isolate the device.

A WAF (B) protects web applications, not device access.

A network IDS (D) only monitors traffic and does not block access.

Thus, C: Host isolation is the correct answer.