Which type of relaxer is known for a strong straightening ability?

Sodium hydroxide relaxers are known for their powerful straightening capabilities. They function through a process that breaks down the disulfide bonds in the hair, which are responsible for wave and curl patterns. This method allows for a more significant alteration of the hair structure, making it highly effective for coarse and resistant hair types that require a strong relaxer for effective straightening.

Sodium hydroxide is highly alkaline, which enhances the relaxer’s ability to penetrate the hair shaft and modify the hair's natural texture. This results in a straighter finish that is lasting, although the process requires careful application and neutralization to avoid damage.

The other types of relaxers focus more on gentler methods or are designed for different purposes. Ammonium thioglycolate, while effective, produces more of a soft wave and is generally used for permanent waves rather than intense straightening. Soft curl relaxers combine both a relaxing and curling process and keratin-based treatments primarily aim to smooth hair without altering its natural curl pattern as dramatically. Thus, sodium hydroxide is distinctly recognized for its strong straightening ability, making it the most appropriate choice for this question.