To solve this problem, we need to understand the processes of transpiration and photosynthesis in plants:$\newline \bullet$Transpiration:$\newline$ - Water vapor exits the plant through stomatal openings during transpiration.$\newline \bullet$Photosynthesis:$\newline$ - Carbon dioxide enters the plant through the same stomatal openings for photosynthesis.$\newline$Now, let's analyze the options:$\newline \bullet$Option 1: Both processes cannot happen simultaneously.$\newline$ - This is incorrect because both processes can occur at the same time.$\newline \bullet$Option 2: Both processes can happen together because the diffusion coefficient of water and CO$_2$ is different.$\newline$ - This is correct. The diffusion coefficients of water vapor and carbon dioxide are different, allowing both to diffuse through the stomata simultaneously.$\newline \bullet$Option 3: The above processes happen only during night time.$\newline$ - This is incorrect because photosynthesis primarily occurs during the day when light is available.$\newline \bullet$Option 4: One process occurs during day time, and the other at night.$\newline$ - This is incorrect because both processes can occur during the day.$\newline$Therefore, the correct option is Option 2: Both processes can happen together because the diffusion coefficient of water and CO$_2$ is different.