Which of the following is the largest unit of energy

The correct option is Calorie.

To determine which of the given options represents the largest unit of energy, let us convert all the units into standard SI units (Joules, $J$).

1. Electron volt ($eV$): An electron volt is the amount of kinetic energy gained by a single electron when accelerating through an electric potential difference of one volt.
1 eV = 1.602 × 10^-19 J

2. Erg: The erg is a unit of energy in the CGS (centimetre-gram-second) system of units.
1 Erg = 10^-7 J

3. Joule ($J$): The Joule is the standard SI unit of energy.
1 Joule = 1 J

4. Calorie ($cal$): A calorie is a unit of thermal energy, defined as the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius.
1 Calorie = 4.184 J

Comparing the values in Joules:
1.602 × 10^-19 J (Electron volt) < 10^-7 J (Erg) < 1 J (Joule) < 4.184 J (Calorie)

Therefore, 1 Calorie is equal to approximately 4.184 Joules, making the Calorie the largest unit of energy among the choices provided.