Arabidopsis thaliana is a small angiosperm related to mustard that is commonly used in plant research experiments. In one experiment, an antifreeze protein from a species of bacteria was inserted into the genome of Arabidopsis thaliana, and the plants were exposed to freezing temperatures. Plants without the antifreeze protein had 90% mortality, while those with the antifreeze protein only had 20% mortality. What was the dependent variable in this experiment?

the percent of the plants surviving

Explanation:

A dependent variable is the one which changes with change or variation in the independent variable.  

An independent variable is the one which is varied in an experiment to study its impact on other factors (dependent variables)

Here in this experiment, the factor which is varied is the antifreeze protein as i t is inserted into the genome of Arabidopsis thaliana to check whether they survive or not.  

Since the plants without antifreeze proteins have lower chances of survival as compared to plants with antifreeze protein.  

Hence, antifreeze protein is the independent variable and % of plants surviving is the dependent variable.

"d.) carbon dioxide and hydrogen sulfide"