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

Instantaneous rate

The correct answer is Option (3) → Instantaneous rate

In chemical kinetics, reaction rate can be described in two ways:

• Average rate $\rightarrow$ over a time interval
• Instantaneous rate $\rightarrow$ at a specific moment

The phrase "at a particular moment of time" directly refers to instantaneous rate.

Option-wise Explanation

1. Order of reaction

Order tells how the rate depends on concentration of reactants. It is a power term in the rate law, not a measure of speed at a given time. So it does not describe the rate at a specific moment.

2. Average rate

Average rate is calculated over a time interval:

$\text{Rate} = \frac{\Delta \text{concentration}}{\Delta t}$

It does not give the rate at one exact instant, only over a duration.

3. Instantaneous rate

This is the rate at a particular instant during the reaction. It is obtained from the slope of the tangent to the concentration–time curve at that point. This exactly matches the definition given in the question.

4. Rate constant

Rate constant ($k$) is a proportionality constant in the rate equation. It depends on temperature and nature of reaction, not on time. It is not the rate itself at a given moment.