A capacitor has a capacitance of one farad (F) if it stores one coulomb of charge when connected across an electromotive force (EMF) of:

The correct answer, which states that a capacitor has a capacitance of one farad when it stores one coulomb of charge at an electromotive force (EMF) of 1 volt, is derived from the fundamental relationship defined by the capacitance formula:

Capacitance (C) is equal to the charge (Q) stored in the capacitor divided by the voltage (V) across it. Mathematically, this is expressed as:

C = Q / V

In this case, if the capacitance is 1 farad and the charge stored is 1 coulomb, we can determine the voltage:

1 F = 1 C / V

This implies that V = 1 C / 1 F = 1 volt. Therefore, the relationship shows that to store 1 coulomb of charge, an EMF of 1 volt is required when the capacitance is 1 farad.

The other options involve values of voltage or current that do not conform to the direct relationship established by this formula. Capacitance is specifically defined in terms of charge per volt, so values like 10 volts, 1 ampere, or 1 milliampere would not correctly represent the scenario put forth in the question concerning