How much sodium chloride must be added to 100 ml of water so that its concentration is 20 parts per million assume that the density of water is 1.00g/ml

2mg

Part per million is unit that equal to mg/kg of water or simply . In this case, you asked how much NaCl needed and know the volume of water(100mL), concentration(20ppm or 20 mg/kg) and water density.
Then, the equation would be

concentration = NaCl weight/ water weight
20 x mg= NaCl weight/ (100ml x 1g/ml)
NaCl weight= 20x  mg/kg x 100g 
NaCl weight= 2000 x g
NaCl weight= 2x g = 2mg