The iron salt used in blue prints is :

Options

A. `FeC_(2)O_(4)`
B. `Fe_(2)(C_(2)O_(4))_(3)`
C. `K_(4)Fe(CN)_(6)`
D. `FeSO_(4)`

Explanation

To solve the question regarding the iron salt used in blueprints, we will analyze the options provided and determine which one is correct based on the characteristics of the compounds.

### Step-by-Step Solution:

1. **Understanding Blueprints**:
- Blueprints are a type of reproduction of technical drawings. They are made using light-sensitive paper coated with iron salts. When exposed to light, these salts undergo a chemical reaction that produces a blue image.

2. **Analyzing the Options**:
- The options given are:
- A) FeC2O4
- B) Fe2C2O4 (thrice)
- C) K4FeCN6
- D) FeSO4

3. **Identifying the Color Producing Compound**:
- The blue color in blueprints is produced due to the presence of certain iron compounds that can absorb light and produce a blue color.

4. **Checking for Unpaired Electrons**:
- Compounds that impart color typically have unpaired electrons. This is because unpaired electrons can transition between energy levels when exposed to light, resulting in color.
- Among the options, we need to check which compound has unpaired electrons.

5. **Evaluating Each Option**:
- **A) FeC2O4**: This compound does not have the required structure to produce a blue color.
- **B) Fe2C2O4 (thrice)**: This compound contains iron in a form that can have unpaired electrons and is known to produce a blue color.
- **C) K4FeCN6**: This is potassium ferrocyanide, which is not typically used for blueprints.
- **D) FeSO4**: While this is an iron salt, it does not produce a blue color in the context of blueprints.

6. **Conclusion**:
- The iron salt used in blueprints is **Fe2C2O4 (thrice)**, as it has the required properties to produce the blue color when exposed to light.

### Final Answer:
The iron salt used in blueprints is **Fe2C2O4 (thrice)**.