The plot of concentration of the reactant vs time for a reaction is a straight line with a negative slope. This reaction follows
(a)zero order rate equation
(b)first order rate equation
(c)second order rate equation
(d)third order rate equation

Options

A. zero order rate equation
B. first order rate equation
C. second order rate equation
D. third order rate equation

Explanation

To solve the question regarding the order of a reaction based on the plot of concentration of the reactant versus time, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Graph**:
- The question states that the plot of concentration of the reactant versus time is a straight line with a negative slope. This indicates that as time increases, the concentration of the reactant decreases linearly.

2. **Analyzing the Slope**:
- A straight line with a negative slope suggests that the rate of change of concentration with respect to time is constant. Mathematically, this can be expressed as:
\[ \frac{d[\text{Reactant}]}{dt} = -k \]
where \( k \) is a positive constant.

3. **Identifying the Order of Reaction**:
- In chemical kinetics, the order of a reaction can be determined from the relationship between concentration and time.
- For a zero-order reaction, the rate of reaction is independent of the concentration of the reactant. The rate law for a zero-order reaction can be expressed as:
\[ \text{Rate} = k \]
- This means that the concentration decreases linearly over time, which matches the description of the graph provided in the question.

4. **Conclusion**:
- Since the plot of concentration versus time is a straight line with a negative slope, it indicates that the reaction follows a zero-order rate equation.

Thus, the correct answer is:
**(a) zero order rate equation.**