Which of the following keyloggers cannot be detected by anti-virus or anti-spyware products?

Which of the following keyloggers cannot be detected by anti-virus or anti-spyware products?

A.
Covert keylogger

B.
Stealth keylogger

C.
Software keylogger

D.
Hardware keylogger

Explanation:
As the hardware keylogger never interacts with the Operating System it is
undetectable by anti-virus or anti-spyware products.