Deficiency of Nitrogen causes Chlorosis:
- Nitrogen is an essential nutrient for plants and is required in large quantities for their growth and development.
- It is a major component of amino acids, proteins, enzymes, chlorophyll, and nucleic acids, which are crucial for various physiological and metabolic processes in plants.
- Chlorosis is a condition characterized by yellowing of the leaves due to a lack of chlorophyll. It is one of the common symptoms of nitrogen deficiency in plants.
- Since chlorophyll is responsible for the green color of leaves and is crucial for photosynthesis, the lack of nitrogen affects the synthesis of chlorophyll, resulting in chlorosis.

Potassium works as an Osmoregulator in plants:
- Potassium is an essential macronutrient required by plants for various physiological processes.
- It plays a vital role in osmoregulation, which is the control of water balance and movement within plant cells.
- Potassium ions (K+) are involved in the regulation of stomatal opening and closing, which affects water loss through transpiration.
- It helps maintain turgor pressure within plant cells, which is necessary for cell expansion and maintaining cell structure.
- Potassium also plays a role in the transport of sugars and nutrients throughout the plant.

Calcium acts as a Second Messenger in plant responses:
- Calcium ions (Ca2+) have a crucial role as second messengers in various signaling pathways within plant cells.
- They are involved in mediating cellular responses to various stimuli, such as hormones, light, pathogens, and environmental stresses.
- Calcium signaling is essential for processes like cell division, growth, differentiation, and response to stimuli.
- Calcium ions are rapidly released into the cytoplasm in response to a specific signal, and their concentration acts as a signal to activate downstream responses, such as gene expression or enzyme activation.

Overall, the correct statements are A) Deficiency of nitrogen causes chlorosis, C) Potassium works in a plant as an osmoregulator, and D) Calcium is a second messenger in various plant responses.