Two clues narrow this question down: the antioxidant has to come from a desert plant, and it has to work better when paired with citric acid than it does on its own.
Maleic acid and BHA can both be ruled out on origin alone, maleic acid is a simple synthetic acid rather than an antioxidant proper, and BHA is a lab-synthesised phenolic compound, not a plant extract. Tocopherols do come from nature, but their usual source is vegetable oil seeds, not anything found in a desert environment.
Nordihydroguaiaretic acid is different. It is isolated from the creosote bush, a shrub adapted to survive in dry desert conditions across the southwestern United States and parts of Mexico. As an antioxidant it scavenges free radicals directly, and when citric acid is added alongside it, the citric acid ties up trace metal ions that would otherwise speed up oxidation, so the two work together more effectively than either does alone. That combined, mutually reinforcing effect is exactly what synergism means.
Matching both the desert origin and the citric acid synergy, NDGA is the correct answer.