The softest rock is

ANS:C - talc

Moh's hardness number be like;

Talc 1
Gypsum 2.
Calcite 3.
....
Topaz 8.
Diamond 10.
So, the talc is the softest and the diamond is hardest. Talc is a clay mineral composed of hydrated magnesium silicate with the greasy feel. Its streak is white. Soapstone is a metamorphic rock composed predominantly of talc.

Formula : Mg3Si4O10(OH)2.
Optical properties: Biaxial (-).
Crystal class : Either monoclinic (2m) or triclinic.
Crystal system : Monoclinic or triclinic.