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

The correct option is Lemongrass.

Explanation:

Lemongrass contains essential oils such as citronella, oil of lemon eucalyptus, and citral, which are natural compounds known to repel mosquitoes and other insects.

Let's look at the options provided:

1. Congress grass (Parthenium hysterophorus): Known as an invasive weed causing allergies and dermatitis rather than a insect repellent.
2. Elephant grass (Pennisetum purpureum): A high-yielding forage crop used mainly as fodder for livestock.
3. Lemongrass (Cymbopogon species): Extensively used to extract oil (citronella oil) for preparing natural mosquito repellent creams, sprays, and candles.
4. Nut grass (Cyperus rotundus): A common perennial weed in agricultural fields.

Therefore, Lemongrass is the correct plant used in preparing a natural mosquito repellent.