Potential security disadvantages of virtualization include:
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VM layer configuration is security relevant
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More intrusive intrusion detection via introspection
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An extra layer for defense-in-depth
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More flexible discovery/eviction of kernel root kits
Virtualization introduces security considerations because the hypervisor (VM layer) becomes a critical security-relevant component that must be properly configured. If the VM layer is misconfigured or compromised, it can expose all guest VMs to risk. Options B and C describe advantages (introspection for detection, defense-in-depth layer), while D is not a standard security disadvantage of virtualization - VM isolation actually makes some kernel-level attacks more contained.
Virtualization adds a whole new software layer (the hypervisor) that itself must be configured and secured correctly — a misconfigured VM layer becomes a new attack surface and a single point of compromise for every guest VM it hosts. The other options actually describe security advantages of virtualization (introspection improves intrusion detection, layering aids defense-in-depth, and hypervisor-level visibility helps detect rootkits), so they are the opposite of a disadvantage.