Question

what is the answer .. ??
How do we know that .. ??

Answer 1

Your answer should be (d).

If you see carefully, except (d), all are strong acids, that means they dissociate completely and at equilibrium only constituent ions are present.

[Eg, HCl ---> H++Cl-]

But in weak acids, dissociation is poor hence at equilibrium both ions and molecules are present, since their dissociation is reversible.

[Ch3cooh<=>ch3coo-+h+]