Since, the dimensions of sandbox are
meter,
meter and
meter.
One cubic meter of the sandbox contains
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.
Since, one cubic meter of the sandbox contains
grains of sand.
Therefore, Number of grains in the sandbox =

=

Option D is the correct answer.