Final answer:
Ammonium thioglycolate is likely the safest option to prevent splits around the hairline among the bases listed, because it is milder than very strong bases like sodium hydroxide or potassium hydroxide, which can cause hair brittleness.
Step-by-step explanation:
The best base to prevent splits around the hairline when treating hair is likely not one of the stronger bases such as sodium hydroxide or potassium hydroxide, as these are very caustic and can cause irritation or burns if not used properly.
While the question does not provide direct information pertaining to the safety or efficacy of the bases for hair treatment, considering the properties of the bases listed, ammonium thioglycolate might be a safer option as it is not as strong as sodium or potassium hydroxide. It is commonly used in hair perm solutions to break disulfide bonds in the hair, which can then be reformed to create curls or waves without the high alkalinity that can cause hair to become brittle and lead to splits.