What is the primary effect of a keratin treatment on hair?

The primary effect of a keratin treatment on hair is to smooth and reduce frizz. These treatments typically involve the application of a keratin-based formula that penetrates the hair shaft, filling in the gaps and helping to seal the hair cuticle. This process leads to a more polished look, decreasing the overall frizz and making the hair more manageable. This is especially beneficial for individuals with curly or wavy hair who struggle with frizz and flyaways, as the treatment can significantly improve the hair's texture, making it look sleek and smooth.

While other options like adding volume, lightening, or creating curls may be desired effects in different contexts or with different hair products, they are not the primary purpose of keratin treatments. The main goal is to provide a sleek and frizz-free finish, making option B the most accurate answer.