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

Lemongrass is the correct answer because it contains citronella oil, a natural essential oil widely used in commercial and traditional mosquito repellents for its ability to mask scents that attract insects. While Congress grass (Parthenium) is a common distractor, it is actually an invasive weed known for causing skin allergies and respiratory issues rather than acting as a repellent. This question tests the core concept of economic botany and the practical application of plant-derived chemical compounds in daily life.