Correct answer: (1) 20 ppm

The concentration of a solution is expressed as parts per million or ppm units.  It is the short version of metric units mg/L.  Therefore, it is known that 1 ppm is equivalent to 1 mg/L.

Answering the question:

We are given the masses of Na₃PO₄ as 0.02 g and H₂O as 1000 g.  The solubility expressed in ppm is actually the mass of the dissolved substance (Na₃PO₄) divided by the volume of the solvent (H₂O).  However, we can not directly proceed with the given data because we need to comply with mg/L unit to arrive with a concentration value in ppm.

First, the mass of Na₃PO₄ should be converted to mg unit.  0.02 g should be multiplied with the conversion factor of 1000 to arrive to the value of 20 mg.

Since we are only given 1000 g as the mass of water, we need to derive its volume in accordance to the given mass.

In order to get the volume, we will take the density of water at standard condition which is 1000 g/L.  Volume is then equal to mass of water divided by the density of water.  We will then arrive at the value of 1 L as the volume.

To get the concentration in ppm, we now divide the mass of Na₃PO₄ in mg unit with the volume of water in L unit.  This will give us the answer of 20 mg/L or 20 ppm which is option no. 1.

Therefore, options 2, 3 and 4 are all eliminated.