Choose the abbreviation of the postulate or theorem that supports the conclusion that the triangles are congruent. given: ∠c, ∠f are rt.∠'s; ac = df; bc = ef

We're given two right triangles (since we're told that angle C and angle F are right angles)

The two legs, ie the sides adjacent to the right angle, for the two triangles are also given to be congruent to one another
leg AC = leg DF
leg BC = leg EF

Because we have two pairs of legs congruent for these two triangle triangles, we use the Leg Leg Theorem which abbreviates to the LL Theorem, or simply LL. 

This is why the answer is choice B) LL