Which signature type results in a legitime alert been dismissed?

This is incorrect.
If the IDS/IPS identifies the traffic as malicious, it is a Positive.
If the traffic is in fact malicious, this is a True Positive.
If the traffic in NOT malicious, this is a False Positive.

If the IDS/IPS identifies the traffic as legitimate, it is a Negative.
If the traffic is in fact legitimate, this is a True Negative.
If the traffic in NOT legitimate, this is a False Negative.

This question is asking the condition for MALICIOUS (legitimate alert) traffic being dismissed.
This means that the engine has identified the traffic as legitimate when it is in fact malicious.
The result is a FALSE NEGATIVE.
This means that the malice test on the traffic showed that the traffic was not malicious (NEGATIVE), but was incorrectly identified this way (FALSE)
The result of the engine classifying the traffic as legitimate despite it being malicious, is a FALSE NEGATIVE