A fuel will have a large fuel value when one gram of it on heating gives more of.

Options

A. `CO_2`
B. `Ash`
C. Water vapours
D. Calories

Answer

To solve the question, we need to determine what a fuel will produce in large quantities when one gram of it is heated, which indicates a high fuel value. Here's the step-by-step solution:

### Step 1: Understand the concept of fuel value
- Fuel value, or calorific value, refers to the amount of heat energy that can be obtained from a specific amount of fuel when it is completely burned. A higher calorific value means that more energy is released during combustion.

**Hint:** Remember that calorific value is a measure of energy released during combustion.

### Step 2: Analyze the options provided
- The options given are:
1. CO2 (carbon dioxide)
2. Ash
3. Water vapor
4. Calories

**Hint:** Think about what products of combustion indicate a high energy release.

### Step 3: Evaluate each option
- **CO2:** While carbon dioxide is produced during combustion, it does not indicate the energy content of the fuel itself.
- **Ash:** Ash is a residue left after combustion and does not contribute to the energy output.
- **Water vapor:** Water vapor is also a product of combustion but does not directly indicate the energy value.
- **Calories:** Calories are a unit of energy. A fuel that produces a large amount of calories upon combustion indicates a high fuel value.

**Hint:** Focus on which option represents energy output rather than by-products.

### Step 4: Conclusion
- The option that indicates a large fuel value when one gram of fuel is heated is **calories**. Therefore, the correct answer is option D.

**Final Answer:** The fuel will have a large fuel value when one gram of it on heating gives more of **calories**.