Add-on security can be defined as:
A.
Physical security that complements logical security measures
B.
Protection mechanisms implemented as an integral part of an information system
C.
Layer security incorporated into an existing system
D.
Protection mechanisms implemented after an information system has become operational
Explanation:
RFC 2828 defines add-on security as "The retrofitting of protection
mechanisms, implemented by hardware or software, after the [automatic data
processing] system has become operational."