The correct answer is option 'C', Horticulture.

Horticulture is the science and art of cultivating fruits, vegetables, flowers, and ornamental plants. It involves the intentional and controlled cultivation of plants for human use, whether it be for food, medicine, or aesthetics. Horticulture is a human-made activity and does not fall under the category of natural vegetation.

Let's break down the different types of vegetation mentioned in the question and understand why horticulture does not belong to the category of natural vegetation:

1. Forests:
Forests are large areas dominated by trees and various forms of vegetation. They occur naturally and are characterized by their dense canopy cover, diverse plant and animal species, and complex ecological interactions. Forests play a crucial role in maintaining the balance of the ecosystem, providing habitat for wildlife, regulating climate, and supplying resources such as timber and medicinal plants.

2. Mangroves:
Mangroves are a type of vegetation found in coastal areas, where land and sea meet. They consist of salt-tolerant trees and shrubs that grow in intertidal zones, where they are subjected to regular tidal inundation. Mangroves are important ecosystems that provide habitat for a diverse range of species, protect coastlines from erosion and storm damage, and act as nurseries for various marine organisms.

3. Horticulture:
Horticulture, as mentioned earlier, is the cultivation of plants for human use. It involves the deliberate selection, breeding, and management of plants to enhance their growth, health, and productivity. Horticultural practices can include growing fruits, vegetables, flowers, herbs, and ornamental plants in gardens, nurseries, and greenhouses. Horticulture is a human intervention and often involves the use of advanced techniques such as grafting, hybridization, and controlled environments to optimize plant growth and produce desirable traits.

4. Grasslands:
Grasslands are ecosystems dominated by grasses and herbaceous plants. They occur in both tropical and temperate regions and can be found in various forms such as savannas, prairies, steppes, and meadows. Grasslands support a rich diversity of plant and animal species and play essential ecological roles such as carbon storage, soil stabilization, and water filtration.

In conclusion, while forests, mangroves, and grasslands are natural vegetation types that occur without human intervention, horticulture is a human-made activity focused on cultivating plants for human use. Therefore, horticulture does not belong to the category of natural vegetation.