Orchids were studied to determine if the amount of humidity affected the flowering of these plants. which of these was the independent variable in this study?

The independent variable is amount of humidity. 

An independent variable is the variable in an experiment that can be changed. In this case, the orchids cannot change, but the humidity can. The aim of the study is to see whether humidity has an affect on the flowering of the orchids, To answer this query, the amount of humidity should be changing, but the orchids not to see if it was the humidity in fact, that caused the changes.  

the amount of humidity 

Independent variable is the parameter that hopefully will affect the dependent variable. 
The orchids are studied to determine if the amount of humidity affected the flowering of these plants. Humidity would be the independent variable while orchids would be the dependent variable.

The right answer is c.sandstone.