Here, AlF_{3} is an ionic compound in which one formula unit of this has three fluoride ions attached to it.

So, in 3 x 10^{24} number of fluoride ions, number of formula units of AlF_{3} is equal to

= (3 x 10^{24}) / 3 = **1 x 10**^{24} Formula Units

**So, option (B) is correct**