Scientists tested the effect of a fertilizer on the growth of plants. they had 2 sets of plants, group a which gets fertilizer, water and sun and group b which gets only water and sun (but in equal amounts to group a). 1. explain how this scenario is a controlled experiment and identify the manipulated variable.

The manipulated variable is the fertilizer since the other factors (water and sunlight) are given in equal amounts.
It is a controlled experiment because one group receives fertilizer while the other does not.  Any differences in growth should be due to either having fertilizer or not.

Note: that if the fertilized plants don't do as well, the experiment was still a success because, it showed that the fertilizer had an effect on the plant (it hindered their growth).  This may not be what the scientist was hoping (if he was working for a fertilizer company trying out a new product), but it is a valid experiment.