The correct answer is option 'C' which states that the energy gained by an electron will be 7 eV (electron volts). Let's understand this answer in detail.

Potential Difference:
- Potential difference, also known as voltage, is the difference in electric potential between two points in an electric circuit.
- It is measured in volts (V) and represents the amount of work done to move a unit charge (such as an electron) from one point to another.

Energy gained by an electron:
- When an electron is accelerated through a potential difference, it gains energy due to the electric field.
- The energy gained by an electron can be calculated using the formula:
Energy (E) = Charge (q) x Potential Difference (V)
where the charge is the charge of an electron (e) and the potential difference is given as 7 volts.

Understanding electron volts (eV):
- Electron volt (eV) is a unit of energy commonly used in atomic and subatomic physics.
- It is defined as the amount of energy gained or lost by an electron when it is accelerated through a potential difference of one volt.
- 1 eV is equal to the charge of an electron (1.6 x 10^-19 coulombs) multiplied by one volt.

Explanation:
- In the given question, the potential difference is 7 volts.
- Using the formula mentioned above, the energy gained by the electron can be calculated as:
Energy (E) = Charge (q) x Potential Difference (V)
= (1.6 x 10^-19 C) x (7 V)
= 1.12 x 10^-18 C.V
= 1.12 x 10^-18 J (since 1 C.V = 1 J)
= 1.12 x 10^-18 / 1.6 x 10^-19 eV (since 1 J = 1.6 x 10^-19 eV)
= 7 eV

Hence, the energy gained by the electron when accelerated through a potential difference of 7 volts is 7 eV.