Final answer:
Razor cutting weakens the cuticle of curly hair, leading to frizz and less defined curls, whereas other methods like texturizing, thinning, and flat ironing do not inherently cause frizz. Proper flat ironing with heat protectant can straighten curls temporarily without long-term damage.
Step-by-step explanation:
Razor cutting should not be used on curly hair because it weakens the cuticle and causes the hair to frizz. The other options, which are texturizing, thinning, and flat ironing, do not inherently cause frizz but it's the method of razor cutting specifically that can be potentially damaging to curly hair. Razor cutting can fray the hair cuticle and disrupt the natural curl pattern, leading to increased frizz and a less defined curl. On the other hand, flat ironing, when done properly with the right heat protectant products, can temporarily straighten the curls without long-term damage.