Which sentence is grammatically incorrect?

The sentence "The cats is playing in the yard." is grammatically incorrect because it features a subject-verb agreement error. In this sentence, "cats" is plural, but the verb form "is" is singular. To correct this error, the verb should agree with the subject in number, so it would instead need to use the plural form "are." Therefore, the correct construction would be "The cats are playing in the yard."

In comparing this option to the others, the other sentences demonstrate proper subject-verb agreement and correct grammatical structure: "The puppies are house trained," "He drank the juice," and "They walk to school every day" all adhere to the rules governing subject and verb forms, ensuring clarity and correctness in communication.