Since, the dimensions of sandbox are 
 meter,
 meter, 
 meter and
 meter and 
 meter.
 meter.
One cubic meter of the sandbox contains 
 grains of sand.
 grains of sand.
We have to determine the grains of sand are in the sandbox.
So, let us determine the volume of the sandbox.
Since, sandbox is in shape of rectangular prism.
Therefore, Volume of sandbox = 

= 

= 
 cubic meters.
 cubic meters.
Since, one cubic meter of the sandbox contains 
 grains of sand.
 grains of sand.
Therefore, Number of grains in the sandbox = 

= 

Option D is the correct answer.