Which of the following is an example of a weed of Kharif season that infests maize crop?
(a) Amaranthus
(b) Water hyacinth
(c) Vallisneria
(d) None of the above

Hint: They are short-lived perennial plants sown in the rainy season. Several species are useful as food crops and are grown both for their leaves and for their edible seeds.

Complete answer:
Kharif season is the season in which the crops are sown in the rainy season (i.e., from June to September) are called Kharif crops. Amaranthus paddy, maize, soybean, groundnut, cotton, etc., are Kharif crops.
Amaranthus is an example of a weed of Kharif season that infests maize crop. Amaranth is a herbaceous plant or shrub that is either annual or perennial across the genus(Amaranthus), of 60–70 species of flowering plants in the family Amaranthaceae, distributed nearly worldwide. They are nutritious pseudocereal (non grass seeds used like cereal grains). Flowers differ interspecifically from the bearing of 3-5 sepals and stamens. Species over the genus possess concentric rings of vascular bundles and make carbon efficiently with a ${C}_{4}$ photosynthetic pathway. Leaves are of an oval or elliptical shape that is either opposite or alternate across species, although most leaves are full and simple with undivided margins.

Additional information:
Other season crops are Rabi and Zayed. In the Rabi season the crops are raised in the winter season (i.e., from October to March) are described as rabi season crops. Examples of rabi season crops are wheat, gram, mustard, pea, etc. In the Zayed season, the crops raised in the summer season are described as Zayed season crops. Examples of Zayed season crops are moong, watermelon, cucumber, etc.
Water hyacinth is a free-floating perennial aquatic plant. Water hyacinth is employed for creating textiles, paper, and for covering fish traps. It ferments rapidly because of its high water content and may supply biomass for biogas production. Vallisneria is the truly classic aquarium plant. Vallisneria is a genus of freshwater aquatic plants, commonly called eelgrass, tape grass, or Vallis.

So, the correct answer is ‘Amaranthus’.

Note:
Amaranthus shows a wide variety of morphological diversity among and even within certain species. Amaranthus is part of the Amaranthaceae that is part of the larger grouping of the Caryophyllales. Although the family is distinctive, the genus has few distinguishing characters among the 75 species present across six continents.