Which of the following is an asbestiform variety of serpentine?

Chrysotile is the only asbestiform variety of serpentine among the choices provided. Serpentine refers to a group of minerals that are typically formed through the alteration of ultra-mafic rocks. Chrysotile, also known as white asbestos, is the most common type of asbestos and has a fibrous, flexible structure which allows it to be easily woven into fabrics and used in various applications. This asbestiform quality makes chrysotile distinct from other mineral varieties that do not exhibit the fibrous crystal habit characteristic of asbestos.

In contrast, the other choices represent different types of amphibole asbestos or non-asbestiform minerals. Riebeckite, for example, is an asbestiform amphibole but does not belong to the serpentine group. Actinolite and cummingtonite-grunerite are also types of amphibole asbestos, meaning they are unrelated to the serpentine family. This distinction is essential for understanding the classification and health risks associated with different asbestos types.