Final answer:
The use of a neutralizing shampoo or stabilizer after a hair relaxer service helps to prevent over-processing by neutralizing remaining chemicals and re-hardening the hair by reforming disulfide bonds.
Step-by-step explanation:
When performing a relaxer service, the key purpose of using a neutralizing shampoo or stabilizer after the hair is free of chemicals and excess moisture is C) Prevent over-processing and re-harden the hair. This step is crucial because the neutralizing shampoo stops the chemical processing by neutralizing any remaining alkaline residues that can continue to break down the hair's structure if left unchecked. During a chemical relaxer service, disulfide bonds within the hair are broken to allow the hair to be reshaped into a straighter form. However, after the hair is thoroughly rinsed from the chemical relaxer, it is in a fragile state and the disulfide bonds need to be reformed to a new, straighter configuration. This is where the neutralizing shampoo comes into play; it reoxidizes the disulfide bonds, firming up the hair's new shape.