Phenyl mercuric acetate
A. Reduces transpiration rate
B. Reduces photosynthesis
C. Kills the plat
D. Reduces respiration

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Hint:Phenyl mercuric acetate is an organometallic compound having mercury. It was used as a preservative in paints. It also works as a disinfectant. It acts as an antitranspirant when applied to plants. It is applied to Christmas trees, newly transplanted shrubs, and cut flowers to preserve and protect them from dying.

Complete answer:Transpiration is a process involved in the leaves of a plant through which the water is evaporated from the plant. Stomata which are pore-like structures over the surface of leaves ensure gaseous exchange. The plants require carbon dioxide for photosynthesis and they release oxygen as a by-product through stomata. Along with the gaseous exchange, the evaporation of water occurs from the surface of leaves. This leads to water loss or drying of plants if it occurs in excess.
To avoid or reduce the rate of transpiration in plants chemical compounds are used called antitranspirants. These are preserving and protecting substances that prevent plants from drying up too quickly. They also protect the leaves from fungal diseases and salt burn. Antitranspirants are of two main types called metabolic inhibitors and film-forming antitranspirants.
Phenyl mercuric acetate is a metabolic inhibitor that reduces the stomatal opening. This increases resistance to water vapor diffusion without disturbing the carbon dioxide uptake by leaves. Other examples of metabolic inhibitors antitranspirants are ABA or Abscisic acid and aspirin.

Hence, option A is the right answer.

Note: The film-forming antitranspirants are more of a use in preserving Christmas trees. They form a colorless layer over the leaf surface. Diffusion of gases is allowed but water vapor diffusion is inhibited. The Christmas trees need to be kept inside for a long time which makes them dry. Hence, film-forming antitranspirants like oil, waxes, silicone, etc. are used.