Four boxes of the same volume are each completely filled with one of the following items: sand, marbles, paperclips, or dice. the order of the boxes from least to greatest mass is as follows: paperclips, dice, marbles, and sand. which box has the greatest desity?

The answer is sand.

A density (D) is a quotient of mass (m) and volume (V):
D = m/V

The volume is the same, so let it be 1, so we can easily assess the relationship between density and mass:
D = m/1
D = m

So, when mass increases, density increases, too.
Therefore:
paperclips, dice, marbles, sand:
     m1        < m2 <    m3    <  m4
     d1        <  d2  <    d3    <  d4