1. Use Einstein’s photoelectric equation

For incident light of wavelength $\lambda$,

where $\phi$ is the work function and $V_s$ is the stopping potential.

It is often convenient to write

where $\lambda_0$ is the threshold wavelength.

2. Write equations for the two cases

Case 1:

When wavelength is $\lambda$, stopping potential is $8\,\text{V}$:

Case 2:

When wavelength is $3\lambda$, stopping potential is $2\,\text{V}$:

3. Eliminate constants by dividing/subtracting

Let

Then the equations become

Subtract the second equation from the first:

Now substitute into the second equation:

4. Check with first equation

Correct.

5. Final answer

Threshold wavelength is

So the correct option is B.