Read the following sentences and find out what kind of adverb the underlined ones are.
We have met before once.

The correct answer is: Adverb of frequency

Solution : Option(b) Adverb of frequency
Adverb of frequency is used to denote how often an action or event is happening . It answers the question "how often". Example- Rarely, never, often, weekly, etc.
Here, in the sentence "We have met before once", "once" answers the question "how often". So, Adverb of frequency.