If an emitter current is changed by 4 mA, the collector current changes by 3.5 mA. The value of $$\beta$$ will be:

We are given that when the emitter current changes by $$\Delta I_E = 4$$ mA, the collector current changes by $$\Delta I_C = 3.5$$ mA.

The current gain $$\alpha$$ (common base) is defined as $$\alpha = \frac{\Delta I_C}{\Delta I_E} = \frac{3.5}{4} = 0.875$$.

The relation between $$\beta$$ (common emitter current gain) and $$\alpha$$ is $$\beta = \frac{\alpha}{1 - \alpha}$$.

Substituting the value of $$\alpha$$, we get $$\beta = \frac{0.875}{1 - 0.875} = \frac{0.875}{0.125} = 7$$.

Hence, the correct answer is Option D.