An acid having basicity one is

Solution (By Examveda Team)

Let's understand what basicity of an acid means first.
The basicity of an acid is simply the number of ionizable hydrogen atoms (H⁺) that one molecule of the acid can donate when it dissolves in water.
Think of it like this: how many "acidic" hydrogens can it give away?

If an acid has basicity one, it means it can donate only one H⁺ ion. We call such an acid a monoprotic acid.
Let's look at the options:

Option A: Phosphoric acid (H₃PO₄)
This acid has three (3) ionizable hydrogen atoms.
So, it can donate 3 H⁺ ions.
Its basicity is three.
Therefore, this option is incorrect.

Option B: Sodium dihydrogen phosphate (NaH₂PO₄)
This is a salt. However, the H₂PO₄⁻ ion in this salt still has two (2) hydrogen atoms that can be donated as H⁺ ions.
So, if it acts as an acid, its basicity would be two.
Therefore, this option is incorrect.

Option C: Disodium hydrogen phosphate (Na₂HPO₄)
This is also a salt. The HPO₄²⁻ ion in this salt has one (1) hydrogen atom that can be donated as an H⁺ ion.
So, if it acts as an acid, its basicity is one.
This matches what the question is asking for!
Therefore, this option is correct.

Option D: Sodium phosphate (Na₃PO₄)
This is a salt. The PO₄³⁻ ion in this salt has no (0) hydrogen atoms to donate as H⁺ ions.
In fact, it's a basic salt.
So, its basicity is zero.
Therefore, this option is incorrect.

So, the correct answer is Disodium hydrogen phosphate (Na₂HPO₄) because it can donate only one H⁺ ion.