Option 1: Manganese is an essential micronutrient for plants, but it does not directly increase the absorption of water and calcium. Manganese plays a role in various enzymatic reactions and helps in photosynthesis.

Option 2: Boron is the correct answer. Boron is essential for the growth and development of plants. It plays a crucial role in the uptake and utilization of water and calcium. It helps in cell wall formation, membrane integrity, and carbohydrate metabolism, which in turn increases water and calcium absorption in plants.

Option 3: Copper is essential for plants, but it does not directly increase the absorption of water and calcium. Copper is involved in processes like photosynthesis and enzyme activation.

Option 4: Molybdenum is essential for plants, but it does not directly affect water and calcium absorption. Molybdenum is involved in nitrogen metabolism and helps in converting nitrate into ammonia for plant use.

Therefore, the correct answer is option 2: Boron increases the absorption of water and calcium in plants.