Q. Which one of the following is used in preparing a natural mosquito repellent?

Explanation : 

Natural Mosquito Repellents: Power of Lemongrass

Lemongrass (Cymbopogon citratus) is commonly used in preparing natural mosquito repellents due to its high citronella content, which is a natural oil that repels mosquitoes. Citronella is often used in mosquito repellent sprays, candles, and oils, and it works by masking scents that are attractive to mosquitoes, thus making it harder for them to locate their targets.

Comparing Grasses: Lemongrass vs. Others

The other grasses listed do not have the same mosquito-repelling properties as lemongrass:

Congress grass, also known as Parthenium hysterophorus, is an invasive weed that can cause allergic reactions in humans and has no known insect-repelling properties.

Elephant grass (Pennisetum purpureum), also known as Napier grass, is a tall grass used for fodder and erosion control, not for repelling mosquitoes.

Nut grass, also known as Cyperus rotundus, is a weed that can be problematic in gardens and agricultural fields and does not have mosquito repellent properties.

Therefore, lemongrass is the correct answer as it is the one known for its use in natural mosquito repellent formulations.