Abdul downloads the hottest new mobile game, Crash of Clans 106. When he installs the game, he checks a box to indicate that he would like to receive promotional emails from the game's developers. Later that day, he receives an email in which he has to click another link to verify his willingness to receive further emails. This process is an example of:

(a) Opt-In
(b) Double Opt-In
(c) Opt-Out
(d) Unsubscribe Confirmation