Answer:
Blocks.
Step-by-step explanation:
As per the question, the gardens exemplify the 'blocks' in the given example as they reflect the experimental sub-group which are identical(similar) to each other and arranged relatively in groups under particular research conditions randomly. Such a research methodology is employed to remove the effects of nuisance variables and reduce the variability of treatment surroundings.
In the given example, the backyard gardens exemplify the sub-divisions or 'blocks' of the subjects of the experiment where the first and second gardeners employ a similar treatment method and condition in order to reduce the variability among the two conditions and produce an effective comparison and estimate between the two. Thus, 'blocks' is the correct answer.