**Cocoons Break to Bring Forth Complete Small Ants**

Cocoons are protective coverings spun by certain insects during their pupal stage. This stage occurs after the larval stage and before the adult stage of an insect's life cycle. Cocoons serve as a shelter for the insect while it undergoes metamorphosis, transforming from a larva into an adult.

When a cocoon breaks, it signifies that the insect inside has completed its transformation and is ready to emerge as an adult. In the case of ants, cocoons are created by the pupae, which are the developmental stage between larvae and adults.

Now, let's take a closer look at the options provided and understand why option B, "Complete small ants," is the correct answer:

**a) Grubs:**
Grubs are the larval stage of beetles. They have a completely different life cycle from ants and do not form cocoons. Therefore, option A is incorrect.

**b) Complete small ants:**
This option is correct because when a cocoon breaks, it brings forth complete small ants. These small ants have undergone metamorphosis inside the cocoon and are now ready to begin their adult life as fully developed ants.

**c) Worker ants:**
Worker ants are a specific caste or type of ant within an ant colony. They are responsible for various tasks such as foraging, nest maintenance, and caring for the brood. While some ants may emerge as workers after their cocoon breaks, not all ants that come out of cocoons will be worker ants. Therefore, option C is incorrect.

**d) Queen ant:**
Queen ants are the reproductive females of the ant colony. They have a different physical appearance and role within the colony compared to worker ants. Queen ants are usually larger and are responsible for laying eggs to continue the ant population. Cocoons do not break to bring forth queen ants. Therefore, option D is incorrect.

In conclusion, when a cocoon breaks, it brings forth complete small ants that have undergone metamorphosis inside the cocoon. These ants are ready to join their colony and fulfill their roles as workers or other castes within the ant society.