Which sentence is grammatically correct?

The chosen sentence is grammatically correct because it uses the verb "lie" in its proper form. "Lie" means to recline or to be in a horizontal position. When instructing someone to assume a resting position, the correct phrase is "lie down." This verb does not take a direct object, as opposed to "lay," which means to place something down and does require an object (e.g., to lay down a book).

In this context, the patient is the one who is being instructed to recline, making "lie" the appropriate choice. Using "lay" or misspellings such as "ley" or "lei" would be incorrect in this context as they either change the meaning of the action or do not exist as relevant verbs in standard English usage.