When 4 volts e.m.f. is applied across a 1 farad capacitor, it will store energy of

The correct answer is B. 4 joules.

The energy stored in a capacitor is given by the formula:

$E = \frac{1}{2}CV^2$

where $C$ is the capacitance in farads, $V$ is the voltage in volts, and $E$ is the energy in joules.

In this case, we have $C = 1\text{ F}$ and $V = 4\text{ V}$. Substituting these values into the formula, we get:

$E = \frac{1}{2}(1\text{ F})(4\text{ V})^2 = 4\text{ J}$

Therefore, the capacitor will store 4 joules of energy.

Option A is incorrect because it is half the correct answer. Option C is incorrect because it is twice the correct answer. Option D is incorrect because it is four times the correct answer.