Which species contains only covalent bonds?

a) $A l {F}_{3}$ b) $N {H}_{4} N {O}_{3}$ c) ${H}_{2} S {O}_{4}$ d) ${K}_{2} C {r}_{2} {O}_{7}$ I know that the answer is c), but could someone please explain why? Thank you so much!

...and only the one distillable liquid. Aluminum fluoride, ammonium nitrate, and potassium dichromate are made of discrete ions...and the parent ions are easy to assess from the formulae: $A {l}^{3 +}$; $A {l}^{3 +} , 3 \times {F}^{-}$; $N {H}_{4}^{+} , N {O}_{3}^{-}$; $2 \times {K}^{+} , C {r}_{2} {O}_{7}^{2 -}$.
On the other hand, sulfuric acid...is the hydrate of a non-metal oxide, an oxyacid; and this is molecular, and thus $d$ is the best answer. Depending where you are you just have to recognize the type of material you got.