If you were going to study the effects of certain fertilizer on plant growth, which list of variables would you most likely want to standardize (keep constant)?

a) Soil type  

b) Water quantity and quality

c) Intensity of light

d) Quantity and quality of fertilizer applied.  

e) Environmental condition

Explanation:

Fertilizer plays an essential role is providing the essential nutrients to plants thereby boosting their growth.  

Hence, in order to conduct a controlled experiment which shall monitor the impact of fertilizer on the growth of plants, all other factors which affect the growth of plants must be standardized or kept constant. These factors include -  

a) Soil type  

b) Water quantity and quality

c) Intensity of light

d) Quantity and quality of fertilizer applied.  

e) Environmental condition

option c is the correct answer.