The correct answer is option 2: Manganese.

Micronutrients are essential nutrients that are required by plants in very small quantities. They play a critical role in various physiological and biochemical processes of plants. Manganese is one of the micronutrients that are necessary for plant growth and development.

Let`s take a closer look at the other options:

Option 1: Calcium is not a micronutrient, but a macronutrient. It is required by plants in larger quantities compared to micronutrients.

Option 3: Magnesium is also not a micronutrient, but a macronutrient. It is required in larger amounts by plants for important processes like photosynthesis.

Option 4: Potassium is not a micronutrient either; it is also a macronutrient. It is required in relatively larger quantities by plants and is involved in various physiological processes.

In conclusion, while calcium, magnesium, and potassium are all essential nutrients for plants, manganese is the micronutrient present in the soil in small quantities for various crops.