When is the best time to perform an antivirus signature update?

A.
Every time a new update is available.

B.
When the local scanner has detected a new virus.

C.
When a new virus is discovered in the wild.

D.
When the system detects a browser hook.

Explanation:
BD
Obvious answer
More reading here
Source: http://www.techrepublic.com/article/four-steps-to-keeping-current-with-antivirus-signature-updates/