Which of the following is the past tense of the verb "lie"?

The past tense of the verb "lie," meaning to recline or be in a horizontal position, is "lay." This form is specifically used when referring to the act of lying down in the past. This distinction is important because "lay" is a past tense that shows completed action, fitting grammatically and contextually when discussing an action that has already occurred.

In contrast, "lain" is the past participle of "lie," used in perfect tenses but not as a simple past form. "Laid" is the past tense of "lay," which means to place something down and is not interchangeable with "lie." Finally, "lying" is the present participle of "lie," indicating an ongoing action rather than a completed one. Thus, "lay" is the correct answer as it directly represents the past tense form of "lie."