The correct answer is: A. the, a
The word “ground” is a common noun, which means that it can refer to any specific or general ground. The word “sky” is a proper noun, which means that it refers to a specific sky, such as the sky above Earth. In this sentence, the speaker is referring to a specific ground and a specific sky, so the correct article to use is “the” for both.
If the speaker was referring to any ground or any sky, then the correct articles would be “a” for both. For example, if the speaker was saying, “I like to lay down on a ground and look up at a sky,” then they could be referring to any ground or any sky, such as a grassy field or a blue sky.
However, in this sentence, the speaker is referring to a specific ground and a specific sky, so the correct articles to use are “the” for both.