What are the properties of alkaline hair products?

The correct choice highlights that alkaline hair products have a swelling effect on the hair shaft. Alkaline substances have a higher pH level, which causes the cuticle, the outer layer of the hair, to lift or open. This lifting allows for better penetration of the product into the hair strands. When the hair shaft swells, it can also enhance the hair's ability to absorb moisture and other beneficial ingredients, as well as facilitate processes like coloring, perming, or chemical relaxing.

The other options do not accurately describe the characteristic effects of alkaline products. While some may make the hair more brittle or affect the cuticle layer, the primary and most notable effect of alkaline products is their ability to swell the hair shaft, which is essential in many hair treatment processes.