Four different experiments were conducted in the following ways:
I. 3 g of carbon was burnt in 8 g of oxygen to give 11 g of `CO_2`.
1.2 g of carbon was burnt in air to give 4.2 g of `CO_2` .
III. 4.5 g of carbon was burnt in enough air to give 11 g of `CO_2`
IV. 4 g of carbon was burnt in oxygen to form 30.3 g of `CO_2`.
Law of constant proportions is not illustrated in experiment(s)

Options

A. I and III only
B. II, III and IV only
C. IV only
D. I only

Explanation

To determine which experiments do not illustrate the Law of Constant Proportions, we need to analyze each experiment based on the law's definition. The Law of Constant Proportions states that a chemical compound always contains its component elements in fixed ratio by mass, regardless of the amount of the compound.

### Step-by-Step Solution:

1. **Understanding the Law of Constant Proportions**:
- The law states that in any given compound, the elements are always combined in the same proportion by mass. For example, in carbon dioxide (CO₂), carbon and oxygen are always present in a fixed ratio of 12:32 (or simplified, 3:8).

2. **Analyzing Experiment I**:
- 3 g of carbon is burnt in 8 g of oxygen to give 11 g of CO₂.
- Here, the mass ratio of carbon to oxygen is 3:8, which is consistent with the Law of Constant Proportions.
- **Conclusion**: This experiment illustrates the law.

3. **Analyzing Experiment II**:
- 1.2 g of carbon is burnt in air to give 4.2 g of CO₂.
- The mass ratio of carbon to CO₂ is 1.2 g of carbon to 4.2 g of CO₂.
- The ratio of carbon to oxygen in CO₂ should still reflect the fixed proportions of 3:8.
- **Conclusion**: This experiment illustrates the law.

4. **Analyzing Experiment III**:
- 4.5 g of carbon is burnt in enough air to give 11 g of CO₂.
- The mass ratio of carbon to CO₂ is 4.5 g of carbon to 11 g of CO₂.
- This ratio can be simplified, and it should still reflect the fixed proportions of carbon and oxygen in CO₂.
- **Conclusion**: This experiment illustrates the law.

5. **Analyzing Experiment IV**:
- 4 g of carbon is burnt in oxygen to form 30.3 g of CO₂.
- The mass ratio of carbon to CO₂ is 4 g of carbon to 30.3 g of CO₂.
- This does not reflect the fixed ratio of carbon to oxygen in CO₂, as the expected mass of CO₂ produced from 4 g of carbon (using the molar ratios) should be different.
- **Conclusion**: This experiment does not illustrate the law.

### Final Answer:
The Law of Constant Proportions is not illustrated in **Experiment IV**.