To increase its revenue, Frankfin, an airline company, decides to promote well$-$known brands in its in$-$flight magazine. These brands would pay a fixed price to Frankfin to print their promotional messages in the magazine. As part of the promotional mix, the in$-$flight magazine is being used for $\_\_\_\_\_.$

a$.$ personal selling

b$.$ public relations

c$.$ direct selling

d$.$ advertising