______ is the rate of a reaction at a particular moment of time.

The correct answer is Option (3) → Instantaneous rate

The rate of a reaction changes as the reactants are used up. The average rate is the change in concentration divided by a time interval, so it holds over the whole interval. To express the rate at a particular moment, the time interval is made very small; this limit is the instantaneous rate:

    \( \displaystyle r_{av} = -\frac{\Delta[\mathrm{R}]}{\Delta t} \qquad r_{inst} = -\frac{d[\mathrm{R}]}{dt} = \frac{d[\mathrm{P}]}{dt} \)

On a concentration–time graph it is the slope of the tangent at that instant.

NCERT (2026-27 edition):
Rate of a Chemical Reaction – “… However, average rate cannot be used to predict the rate of a reaction at a particular instant as it would be constant for the time interval for which it is calculated. So, to express the rate at a particular moment of time we determine the instantaneous rate. It is obtained when we consider the average rate at the smallest time interval say dt ( i.e. when Δt approaches zero).”