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Milk of lime is a common name for calcium hydroxide suspension in water. When it is mixed with copper sulfate (CuSO₄), it produces a chemical mixture known as Bordeaux mixture. This mixture has been used for more than a century as a powerful fungicide in agriculture. It was first discovered in the 19th century by the French scientist Pierre-Marie-Alexis Millardet while searching for a way to protect vineyards from fungal diseases.

Bordeaux mixture works by forming an insoluble layer of copper hydroxide on the surface of plant leaves. The copper ions released from the mixture are toxic to many species of fungi and molds, preventing their growth and spread. Because of this property, it is widely used to control plant diseases like downy mildew, leaf spot, and blight in crops such as grapes, potatoes, tomatoes, and citrus fruits.

Other options listed in the question do not serve this purpose in combination with milk of lime. Chlorine is a disinfectant mainly used in water treatment. Silver nitrate has antiseptic properties but is not used with lime for agricultural fungicidal purposes. Nitric acid is a strong acid and does not act as a fungicide in this context.

The effectiveness of copper sulfate with milk of lime lies in its ability to adhere well to plant surfaces, remaining active even after rain. This makes Bordeaux mixture one of the earliest and most successful chemical protectants in plant pathology. It also represents a classic example of how inorganic compounds are applied in practical life to solve agricultural problems.

Thus, the chemical used with milk of lime to kill fungus and molds is copper sulfate (CuSO₄), forming Bordeaux mixture, an important fungicidal preparation in inorganic chemistry.