Coordinate bonds between molecules
Option C is correct.

You could start by looking up the definition of a coordinate bond.

Here's a tiny excerpt from the above link:


  A coordinate bond is a kind of 2-center, 2-electron covalent bond in which the two electrons derive from the same atom.


In your case, as you have correctly noted yourself, the $\ce{NH3}$ molecule will have an "lone pair" of electrons. However, your judgement that $\ce{HF}$ has none is quite incorrect– the fluorine atom has $3$ lone pairs. (not that it's very relevant to the question, but you can look up the structure of $\ce{HF}$ here)

Since the $\ce{NH3}$ is a lewis base, its lone pair of electrons would abstract the $\ce{H}$ from $\ce{HF}$.



$\ce{NH4+}$ has four equivalent bonds. You could call one of them the "dative" bond, but there's really no way to distinguish them from each others in general cases.

The concept of dative bond comes here just because the bond between nitrogen and the hydrogen from $\ce{HF}$ involves 2 electrons (like all single bonds), both of which are contributed by the nitrogen atom (unlike most single bonds). 

(Image source)