Final answer:
The correct answer is Nitrate (C), as it is an anion rather than a cation, unlike the other options given which are all cations and essential nutrients absorbed by plants.
Step-by-step explanation:
C) Nitrate. This option is not a cation but an anion, which contrasts with the others listed, that are cations and act as nutrients for plants.
We can clarify that calcium (Ca2+), potassium (K+), and magnesium (Mg2+) are all essential macronutrients that plants absorb from the soil as cations. These nutrients provide a range of benefits essential for plant growth and development. Calcium plays a vital role in regulating nutrient transport and supporting many enzyme functions. Potassium contributes to strong stem growth, efficient water movement in plants, and the promotion of flowering and fruiting. Magnesium is at the heart of the chlorophyll molecule, the compound that enables photosynthesis, thus playing a crucial role in the plant’s ability to convert light into energy.
Conversely, nitrate (NO3−), despite being a key nutrient for plant growth as it is part of the nitrogen cycle and necessary for protein synthesis and cell formation, it is an anion. This distinction excludes it from the category of cations beneficial to plants. Hence, nitrate does not fit the criteria requested in the question and is the correct answer.