1. Use the threshold condition for photoelectric emission

   The longest wavelength that can just eject photoelectrons is the threshold wavelength $\lambda_0$.

   At threshold, $h\nu_0 = \phi$ where $\phi$ is the work function.

   Since $\nu = \dfrac{c}{\lambda}$, $\phi = \frac{hc}{\lambda_0}$ so, $\lambda_0 = \frac{hc}{\phi}$

2. Substitute known values

   Given: $\phi = 3.0\,\text{eV}$

   Use the convenient relation: $hc \approx 1240\,\text{eV·nm}$

   Therefore, $\lambda_0 = \frac{1240}{3.0}\,\text{nm}$

3. Calculate

   $\lambda_0 \approx 413.3\,\text{nm}$

   So the longest wavelength is approximately $\boxed{414\,\text{nm}}$

4. Check options

   • A: $215\,\text{nm}$ ✗
   • B: $400\,\text{nm}$ ✗
   • C: $414\,\text{nm}$ ✓
   • D: $200\,\text{nm}$ ✗

Hence, the correct option is C.