Which weak acid has the strongest conjugate base?

A weaker an acid, the stronger its conjugate base, and vice versa for bases. The extent of acidity is measured by the pka or an acid. The higher the pka, the weaker the acid; thus the stronger its conjugate base. You find pka by taking -log(ka), but since you're just measuring them against each other, you really dont need the numbers. The important thing to know is that there is an inverse relationship between ka and pka, so if the larger pka gives you the weakest acid, you're going to look for the smallest ka to provide you with the weakest acid; and this acid will have the strongest conjugate base. Here the lowest ka is that of acetic acid, so this molecule will give you the strongest conjugate base.