What is the simple subject in this sentence? the big blue boat sailed across the lake. a. boat b. sailed across c. lake d. big blue boat

The subject of a sentence is who/what the sentence is about.
In the case of action verbs, it's the one doing the action. (the dog barks)
In the case of linking verbs, it's the one being described. (the cat is nice)

In our sentence, "the big blue boat sailed across the lake," the subject is the big blue boat. When we provide the simple subject, we don't include any modifiers; it would just be A. boat.