The imperative 'Please ________ down' requires the base form of the verb. 'Lie' is correct because it's the base form of the verb meaning to recline or place oneself in a horizontal position. 'Laid' is past tense of 'lay' (to put something down). 'Lay' is present tense but means to put something somewhere, not to recline oneself. 'Lies' is third-person singular present tense, which doesn't fit the imperative structure.