The figure shows the distribution of kinetic energy among the molecul…

The correct answer is Option (2) → The sum of the fractions of all the molecules of the gas, which is equal to one

The y-axis of the figure is the fraction of molecules, N_E/N_T, having a given kinetic energy E, where N_T is the total number of molecules. The area under the curve adds up these fractions over all the energies, so every molecule is counted once: the sum is N_T/N_T = 1, the total probability.

Why the other options are incorrect

Option (1) The fraction of molecules having the most probable kinetic energy — Incorrect. This fraction is the height of the curve at its peak, a single point, not the whole area.

Option (3) The fraction of molecules having kinetic energy higher than the most probable kinetic energy — Incorrect. This is only the part of the area to the right of the line from the peak; the total area also includes the molecules to its left.

Option (4) The total kinetic energy of all the molecules of the gas — Incorrect. The curve plots fractions of molecules, not energies; adding the fractions counts the molecules and gives 1, not their total energy.

NCERT (2026-27 edition):
3.4 Temperature Dependence of the Rate of a Reaction – “According to them, the distribution of kinetic energy may be described by plotting the fraction of molecules (N_E/N_T) with a given kinetic energy (E) vs kinetic energy (Fig. 3.8). Here, N_E is the number of molecules with energy E and N_T is total number of molecules.”

“The area under the curve must be constant since total probability must be one at all times.”

Fig. 3.8: Distribution curve showing energies among gaseous molecules