The correct answer is Lemongrass

Lemongrass:

• Cymbopogon citratus or lemongrass is a tall perennial fast-growing grass with a tuft of lemon-scented leaves from the annulate and sparingly branched rhizomes.
• It grows to a height of 1 m and a width of 5–10 mm and has distinct bluish-green leaves which do not produce seed.
• Lemongrass oil (Cymbopogon citratus) is an effective repellent against mosquitoes and house flies. Hence statement 3 is correct.

• Congress Grass:
  • Congress grass or parthenium, a native of tropical America, came to India accidentally in 1955.
  • A rapidly growing invasive species, this grass competes well for nutrients and space and reproduces fast.
  • One of the world's seven most notorious weeds, parthenium has already taken over about five million hectares of the country's land.
• Elephant grass:
  • Pennisetum purpureum, also known as Napier grass or elephant grass is a robust perennial grass widely naturalized in tropical and subtropical regions of the world.
  • It is included in the Global Compendium of Weeds where it is listed as an agricultural and environmental weed as well as an invasive species.
• Nut grass:
  • Cyperus rotundus, or Nutgrass, is a perennial, glossy-green, grass-like Eurasian sedge or weed with an erect triangular stem branching into three stems of purple, antenna-like seedpods.