1. Identify the role of the acid mixture in nitration

   Benzene nitration occurs using the nitrating mixture of concentrated $HNO_3$ and concentrated $H_2SO_4$.

   The actual electrophile that attacks benzene is the nitronium ion, $NO_2^+$.

2. Formation of the electrophile

   The first step is protonation of nitric acid by sulfuric acid:

   $HNO_3 + H_2SO_4 \rightarrow H_2NO_3^+ + HSO_4^-$

   Here:

   • $H_2SO_4$ donates a proton, so it acts as an acid.
   • $HNO_3$ accepts a proton, so it acts as a base.

3. Generation of nitronium ion

   The protonated nitric acid then loses water:

   $H_2NO_3^+ \rightarrow NO_2^+ + H_2O$

   Thus, the nitrating species is formed.

4. Conclusion about acid-base roles

   In the mixed acid system:

   • $H_2SO_4$ acts as an acid
   • $HNO_3$ acts as a base

5. Evaluate options

   • A: both act as bases  Incorrect
   • B: $HNO_3$ acid, $H_2SO_4$ base  Incorrect
   • C: both act as acids  Incorrect
   • D: $HNO_3$ acts as a base and $H_2SO_4$ acts as an acid  Correct

Therefore, the correct answer is:

$\boxed{D}$