The work functions of six metals are: caesium 2.1 eV, potassium 2.3 eV, lithium 2.9 eV, zinc 4.3…

Step 1. Zinc. $\lambda_0 = \dfrac{hc}{\phi} = \dfrac{1240\ \text{eV nm}}{4.3\ \text{eV}} = 288\ \text{nm}$. Zinc needs ultraviolet light.

Step 2. The photon energy. $E = \dfrac{1240\ \text{eV nm}}{410\ \text{nm}} = 3.02\ \text{eV}$.

Step 3. Which metals. Electrons leave if $E > \phi$. Caesium ($2.1$), potassium ($2.3$) and lithium ($2.9$) are below $3.02$ eV. Zinc, copper and platinum are above it. So $3$ metals emit electrons.

Step 4. Caesium. $K_{\max} = 3.02\ \text{eV} - 2.1\ \text{eV} = 0.92\ \text{eV}$.