Final answer:
Grouping similar experimental units together and randomly assigning them within each group to a treatment is called blocking, which is used to reduce variation and control for certain variables in experimental designs.
Step-by-step explanation:
Grouping together similar experimental units and then randomly assigning the experimental units within each group to a treatment is called blocking. The correct answer is option (d) Blocking.
Blocking is a technique used to reduce variation in experimental design by creating groups (blocks) that are similar in some way that is important to the response of the treatments. Within each block, experimental units are then randomly assigned to treatments. This helps in controlling for the effects of the identified variable, leading to more accurate and reliable results when evaluating the main effects of the treatments being tested.