The correct answer is option 'A' - Fluorine.

Fluorine is one of the most harmful air pollutants for plants, causing significant damage to their growth and development. Here are the reasons why fluorine has the most detrimental effects on plants:

1. Toxicity: Fluorine is highly toxic to plants, even at low concentrations. It can penetrate the plant tissues and disrupt various physiological processes, leading to severe damage.

2. Cellular Damage: Fluorine ions can enter plant cells and interfere with important metabolic reactions, causing damage to cellular structures. This can lead to impaired cell function, reduced photosynthesis, and ultimately, stunted growth.

3. Nutrient Imbalance: Excessive exposure to fluorine can disrupt the uptake and utilization of essential nutrients by plants. It can interfere with the absorption of minerals such as calcium, magnesium, and potassium, leading to nutrient imbalances and deficiencies. This can further impair plant growth and development.

4. Leaf Chlorosis: High levels of fluorine can cause chlorosis, which is the yellowing or whitening of plant leaves. Chlorosis occurs due to the disruption of chlorophyll synthesis, resulting in reduced photosynthetic activity. As a result, plants are unable to produce sufficient energy for growth, leading to weakened overall health.

5. Tissue Necrosis: Fluorine toxicity can also induce tissue necrosis, causing the death of plant cells and tissues. This can result in the formation of brown or black spots on the leaves, stems, or fruits. Necrotic tissues are non-functional and cannot contribute to plant growth or productivity.

6. Reduced Reproduction: Fluorine exposure can negatively impact plant reproduction by affecting flower development, pollen viability, and seed formation. This can lead to reduced seed germination rates and overall decline in plant population.

Overall, the toxic effects of fluorine on plants are wide-ranging and can severely impair their growth, development, and reproductive capacity. It is important to minimize fluorine emissions and implement effective air pollution control measures to protect plant health and ecosystem sustainability.