Which of the following is most soluble in water?

${\mathrm{Ag}}_3{\mathrm{PO}}_4\left[\mathrm{s}\right]\rightleftharpoons {\mathrm{Ag}}_3{\mathrm{PO}}_4\left[\mathrm{aq}\right]\to \underset{3\mathrm{S}}{3{\mathrm{Ag}}^{+}}\left[\mathrm{aq}\right]+{\underset{\mathrm{S}}{{\mathrm{PO}}_4}}^{-3}\left[\mathrm{aq}\right]$

${\mathrm{K}}_{\mathrm{sp}}=(3\mathrm{S}{)}^3\times \mathrm{S}$

⇒ 1.8 x 10^-18 = 27 x S⁴

⇒ 20 x 10^-16 = 3 x S⁴

Solubility, S will be of the order of 10^-4 which is the largest among other option.