Which of the following shows the correct sequence of acidic strength? 

HClO₄ > HBrO₄ > HIO₄

The correct answer is option 4. HClO₄ > HBrO₄ > HIO₄.

The strength of an acid is determined by its ability to donate a proton \((H^+)\) in solution. In general, stronger acids have weaker conjugate bases, meaning that the conjugate base formed after the loss of a proton is less stable.

Let us analyze the given acids:

1. Perchloric acid \((HClO_4)\): This is a strong acid, as perchlorate ion \((ClO_4^-)\) is a highly stable and weakly basic conjugate base. The oxygen atoms in perchlorate ion are highly electronegative, leading to resonance stabilization and delocalization of the negative charge over multiple oxygen atoms, resulting in a highly stable conjugate base.

2. Perbromic acid \((HBrO_4)\): This acid is weaker than perchloric acid but stronger than iodic acid. Bromate ion \((BrO_4^-)\), the conjugate base of perbromic acid, is less stable than perchlorate ion due to the larger size of the bromine atom compared to chlorine. However, it is still more stable than the conjugate base of iodic acid.

3. Periodic acid \((HIO_4)\): This is the weakest acid among the three. The conjugate base of periodic acid, iodate ion \((IO_4^-)\), is the least stable among the three halogenates due to the large size of the iodine atom and the weak electronegativity of iodine, leading to poor resonance stabilization and higher basicity.

Therefore, the correct sequence of acidic strength is \(HClO_4 > HBrO_4 > HIO_4\). As we move down the group from chlorine to iodine, the acidic strength decreases due to the decreasing stability of the conjugate base.