1. Identify what is being asked

We need the technique not used to spot components on a thin layer chromatography (TLC) plate after separation.

2. Recall common methods of visualizing TLC spots

After development of a TLC plate, spots can be detected by:

• Exposure to iodine vapours: many organic compounds absorb iodine and appear as brown spots.
• U.V. light: compounds that absorb UV or plates containing fluorescent indicator help visualize spots.
• Spraying with an appropriate reagent: for example, ninhydrin, sulfuric acid, anisaldehyde, etc.

These are all standard post-development visualization techniques.

3. Check each option

Option A: $I_2$ (Solid)

Iodine crystals are used to generate iodine vapours for visualization of spots.

• Used in TLC spotting/visualization.
• Hence, this is a valid technique.

Option B: U.V. Light

TLC plates are commonly observed under UV light.

• Used in TLC visualization.
• Hence, this is a valid technique.

Option C: Visualisation agent as a component of mobile phase

A visualization agent is generally not added as part of the mobile phase for spotting separated components on a TLC plate. Visualization is usually done after development, not by incorporating the visualizing agent into the solvent system as a standard spotting technique.

• Not a standard technique for spotting components on TLC plate.
• Hence, this is the correct answer.

Option D: Spraying of an appropriate reagent

Spraying the plate with suitable reagents is a standard method.

• Used in TLC visualization.
• Hence, this is a valid technique.

4. Conclusion

The technique that is not used is:

$\boxed{\text{C: Visualisation agent as a component of mobile phase}}$

5. Comparison with stored correct answer

Stored correct answer = C

My derived answer = C

So, they agree.