A veterinarian interested in studying the causes of enteroliths in horses suspects that a diet high in alfalfa may be a cause. The veterinarian decides to use the following design: identify 30 horses at high risk for enteroliths and divide them into groups 10 groups of 3 horses. Some groups consisted of horses in barn stalls, some groups consisted of horses in outdoor paddocks, and some groups consisted of horses in pasture. Within each group one horse is randomly assigned to an alfalfa diet, one is assigned to a grass hay diet, and one horse is fed oat hay. When a horse is in pasture all the other horses in pasture will also receive the diet of the study horse. Therefore, only one horse could be selected from the same pasture. This study has:

Randomized block design

Step-by-step explanation:

A randomized block design is essentially an experimental design where the units of the experiment are in groups which are called blocks. Now, treatments are usually randomly allocated to the units of experiment inside each block and when all the treatments appear a minimum of one time in each block, we can conclude that we have achieved a completely randomized block design.

Now, in the question we see that the experimental units which is the "30 horses at high risk for enteroliths" are divided into 3 groups. We can call these groups blocks. Now we see that further treatments such as alfalfa diet, grass hay diet and oat hay diet are assigned to different horses each within those groups/blocks.

Now, this description fits the definition of a randomized block design given above.

Thus, the study is a randomized block design