Answer:
D
Step-by-step explanation:
All three of A, B and C are required to harden a host. Thus, All of the above makes the best option.
Hardening simply refers to making your data impossible to be penetrated. More like bulletproof.
To do this, ensure measures like
- Regular Program cleanup
- Patches management
- service pack installation
- service or usage policies
...among others.
Cheers