Solution:

Weeds are plants that grow in unwanted locations and compete with cultivated crops for resources such as nutrients, water, and sunlight. They can have a negative impact on crop yield and quality. Among the options given, the weed is Parthenium (option C).

Parthenium (Parthenium hysterophorus):
- Parthenium, also known as Congress grass or carrot weed, is an invasive weed native to the Americas.
- It is a highly adaptable plant that can grow in a variety of climates and soil types, making it a common weed in many countries, including India.
- Parthenium has small, white, and inconspicuous flowers that produce a large number of seeds.
- The plant spreads rapidly and can quickly colonize large areas, outcompeting native plants and reducing biodiversity.
- It is known to cause various health problems in humans and animals, including skin allergies, respiratory issues, and livestock poisoning.
- Parthenium is difficult to control and eradicate due to its ability to regenerate from fragments and its resistance to many herbicides.

Other Options:
a) Sudan grass:
- Sudan grass (Sorghum sudanense) is a fast-growing annual grass used for forage and as a cover crop.
- It is not considered a weed but a cultivated crop.

b) Cowpea:
- Cowpea (Vigna unguiculata), also known as black-eyed pea, is a legume cultivated for its edible seeds.
- It is not a weed but a widely grown crop in many parts of the world.

d) Linseed:
- Linseed, also known as flaxseed, is the seed of the flax plant (Linum usitatissimum).
- It is cultivated for its oil-rich seeds and is not considered a weed.

Therefore, among the given options, Parthenium (option C) is the weed.