The correct equation that represents the relationship between the standard cell potential and the equilibrium constant for the cell reaction is:
(3) $E_{\text {cell }}^{\circ}=\frac{0.0592}{n} \log _{10}(K)$
The general relationship between the standard cell potential $E_{\text {cell }}^{\circ}$ and the equilibrium constant $K$ is given by the Nernst equation, specifically in its standard form:
$E_{\mathrm{cell}}^{\circ}=\frac{0.0592}{n} \log _{10}(K)$