Which of the following pairs of chemical elements and their symbols is INCORRECT?

Potassium - Po

The correct answer is option 2. Potassium - Po.

Let us break down each pair:

1. Phosphorus - P:

This is correct. The chemical symbol for phosphorus is P.

2. Potassium - Po:

This is incorrect. The correct chemical symbol for potassium is K, not Po. The symbol "Po" represents polonium.

3. Iron - Fe:

This is correct. The chemical symbol for iron is Fe.

4. Iodine - I:

This is correct. The chemical symbol for iodine is I.

So, the pair that is INCORRECT is 2. Potassium - Po. The correct symbol for Potassium is K.