Opening and closing of stomata is controlled by
A. Abscisic acid
B. CO2 concentration
C. Light intensity
D. All of the above.

The opening and closing of stomata is primarily controlled by a combination of factors, including:

A. Abscisic acid (ABA): Abscisic acid is a plant hormone that plays a key role in promoting the closing of stomata. When a plant is under stress, such as during drought or high salinity, ABA levels increase, leading to stomatal closure to reduce water loss.

B. Carbon dioxide (CO2) concentration: Stomata tend to open in response to low CO2 levels in the leaf's interior. This allows the plant to take in more CO2 for photosynthesis when it is needed.

C. Light intensity: Light intensity also influences stomatal opening. In the presence of light, stomata often open to allow for photosynthesis, as the plant requires CO2 for this process.

So, the correct answer is D. All of the above. Stomatal opening and closing are regulated by multiple factors, including ABA, CO2 concentration, and light intensity, which work together to ensure that the plant efficiently balances gas exchange and water conservation.