18) C

Firstly, we need to know what a diprotic acid is. A diprotic acid is an acid that can donate two protons per molecule to an aqueous solution.
We know the acid can donate 2 protons. We know NaOH is only able to accept 1 proton.

So we need twice the volume of NaOH to neutralise the acid.

We know that the concentration of NaOH is half that of the acid. In order to match the concentration and the volume, we need 4 times the amount of NaOH to neutralise 30cm³ of the acid.

Volume needed = (30 x 4) = 120cm³