When an alkali is added to an orange red solution containing dichromate ions, a yellow solution is obtained due to the formation of chromate ions.

Adding an alkali to an orange-red solution of dichromate (Cr2O7^2-) results in the formation of chromate (CrO4^2-). This reaction can be explained by the following steps:

1. Acidic dichromate solution: The initial orange-red solution contains dichromate ions (Cr2O7^2-) in an acidic medium. The dichromate ion is a strong oxidizing agent.

2. Alkali addition: When an alkali is added to the solution, it reacts with the acidic medium and neutralizes the excess H+ ions present. This results in an increase in pH (alkaline conditions).

3. Formation of chromate: In an alkaline medium, the dichromate ion (Cr2O7^2-) undergoes a disproportionation reaction. It gets reduced to chromate ion (CrO4^2-) and chromium(III) hydroxide (Cr(OH)3). The reaction can be represented as follows:

2Cr2O7^2- + 3H2O → Cr2O4^2- + 4OH^- + 3O2

The chromate ion (CrO4^2-) is yellow in color and gives a yellow solution. This is the final product formed when an alkali is added to the orange-red dichromate solution.

Explanation:
- The initial solution contains dichromate ions (Cr2O7^2-) in an acidic medium.
- The addition of an alkali neutralizes the acidic medium and increases the pH.
- In an alkaline medium, the dichromate ion undergoes a disproportionation reaction.
- The dichromate ion is reduced to chromate ion (CrO4^2-) and chromium(III) hydroxide (Cr(OH)3).
- The chromate ion gives a yellow solution.

The correct answer is option C, chromate.