Right answer: Selenite gypsum
The hardness is the opposition offered by materials to alterations such as penetration, abrasion, scratching, cutting, and permanent deformations among others. From this fact several scales of hardness of the materials and minerals have been created, being one of the most used the Mohs scale of mineral hardness.
This scale goes from 1 to 10 (where 1 is very soft and 10 very hard) and is based on the principle that any substance can scratch other softer, without the opposite happening.
Then, knowing the hardness of a material we can know if it is most likely become rounded at the fastest rate when tumbled along a stream bottom.
Now, according to Mohs scale of mineral hardness:
-Garnet hardness is 6,5 to 7,5 Mohs
-Plagioclase feldspar hardness is 6 to 6.5 Mohs.
-Pyroxene hardness is 5 to 6,5 Mohs
- Selenite gypsum hardness is 1,5 to 2 Mohs
According to this list above, the the Selenite gypsum least hard, hence it is most likely become rounded at the fastest rate when tumbled along a stream bottom