Answer : 4. "By systemic insecticides"
Explanation :

Prevention: When the white fly reaches the economic limit (6 adults per leaf), apply 300 EC or Oxydemethan Methyl 25 EC (Metasystax) and one liter of Neem based insecticide (Nimbicidin/Ashkar) in 250 liters of water alternately per acre. Spray in.