Which of the following compounds has the highest pka value? a)ccl3ch2nh2 b)cf3ch2nh2 c)cf3nh2 d)chf2chfnh2

The lower the pKa of a Bronsted acid, the more easily it gives up its proton. The higher the pKa of a Bronsted acid, the more tightly the proton is held, and the less easily the proton is given up.

Here we need the highest pKa, so we need to see which compound  will less likely to give proton or hydrogen ion.

Now, all Nitrogen contains a lone pair. But HALOGEN groups( F, Cl, only) being electronegative than NITROGEN [electronegativity of N=3, F=4 and Cl=3], pulls electron pair towards itself. 

The more the lone pair of nitrogen is pulled, the more strong bond between N and H will become, which means less likely to give hydrogen ion.
means high Pka

C) option is the answer because it has 3 F very close to N.