One challenge marketers face when Apple users opt out of activity tracking across mobile apps is$...$

a$.$

Marketers must pay a premium additional fee to access social media platforms' data about Apple users who opt out of activity tracking across apps.$$

b$.$

Marketers cannot advertise to Apple users who opt$-$out of tracking across apps.$$

c$.$

Marketers face all of these challenges when Apple users opt out of tracking their activity across apps.$$

d$.$

Marketers risk advertising to Apple users who have already purchased the promoted product.