Answer
1.65 eV.
Work Step by Step
The minimum work function corresponds to the energy of the least energetic visible light photon, i.e., the one with the longest wavelength.
$$\phi=\frac{hc}{\lambda}=\frac{(4.136\times10^{-15}eV \cdot s)(3.00\times10^8 m/s)}{750\times10^{-9} m}=1.65 eV$$
