What is a common effect of using a hair straightener?

Using a hair straightener primarily serves the purpose of straightening hair, making it easier to manage and style. The heat applied through the straightening process alters the hair's structure temporarily, smoothing out the cuticles and making the hair appear sleek and straight. This transformation can significantly enhance manageability, especially for those with naturally curly or frizzy hair types, allowing for easier styling and a polished look.

The other effects mentioned in the answer choices are not typical results of using a hair straightener. For instance, straighteners do not add moisture to hair; rather, they can sometimes lead to dryness due to the heat. Additionally, using a straightener would generally decrease curl retention rather than increase it, and deep cleaning the scalp is unrelated to the use of a hair straightener, as that involves different hair care practices like shampooing and exfoliating.