B. Gas exchange during the daytime.
Explanation:
There are a couple of buzzwords in this question. The first is homeoestasis. Homeostasis is the "state of steady internal, physical, and chemical conditions maintained by living systems." I know this may seem like a lot of words, so let's break it down into something easier -> Homeostasis is the state of steady conditions maintained by living systems. Easier to handle? Here's an example of homeostasis to really drive it home. When you're hot, you might have noticed that you start sweating. This is homeostasis! Your body temperature is rising, and leaving the steady conditions needed to keep you healthy. Therefore, you produce sweat, which helps cool you and lower your body temperature.
Let's move on to the other buzzword: stoma. Stomas are little openings along the epidermis of plants. The epidermis is the outer layer of leaves- it's thin, and almost like the plant's skin. Stomas' main functions are to regulate the exchange of carbon dioxide and water between the plants and the atmosphere. A plant's main purpse is photosynthesis- without it, the plant would die. Therefore, whenever the plant notices that it doesn't have enough of something to perform photosynthesis, it looks for outside help. This is where the stoma come in. Usually, the stoma help the plant gain more Carbon Dioxide, or help the plant lose water to make sure phtoosynthesis can happen correctly.
With this in mind, we know the answer is either B or D. Here's why it's B. I said before that the stoma's function is to assist in photosynthesis. Photosynthesis is guided by the sunlight, and therefore the sun. The sun is only present during the daytime. So, there's no need for the stoma to function during night. In fact, stoma usually close during the night!
I hope this helps you, and please feel free to ask if you want more clarification of any topic!
