Final answer:
Phone calls are tracked to an advertiser's account using a unique identifier, such as a specific phone number assigned to the ad campaign.
Step-by-step explanation:
When an advertiser uses phone call conversion tracking feature, the calls are tracked to the advertiser's account using a unique identifier. This unique identifier can be a phone number specifically assigned to the ad campaign. Each time a call is made to that phone number, it is recognized as a conversion and attributed to the advertiser's account.
For example, let's say an advertiser is running a campaign with a specific phone number assigned for tracking purposes. When a user sees the ad and makes a call to that phone number, the call will be tracked as a conversion for the advertiser's account.
Therefore, the correct answer to this question is option c: By unique identifier.