Sara Sushi's app, which is installed on mobile devices, notifies customers when they are within half a mile of the restaurant, displays the special offering of the day, lists the user's social media contacts who are in the restaurant, and offers a $10$ percent discount for purchases in the next hour. The strategy employed in the app is an example of which of the following?

a$.$ Ad serving

b$.$ Mass marketing

c$.$ SoLoMo marketing

d$.$ Public relations