When any substance (except water) is heated, its density:
$
  {\text{A}}{\text{. increases}} \\
  {\text{B}}{\text{. decreases}} \\
  {\text{C}}{\text{. remains same}} \\
  {\text{D}}{\text{. none of these}} \\
 $

Hint: Density is the ratio of mass and volume of the substance. So the change caused by temperature on mass and volume should be checked, to now the effect on density.

Complete step-by-step answer:
Mass is a property that is fixed. It does not change unless relativity applies. So, on increasing temperature, the mass remains unaffected.
In general, all substances (except water) expand on heating. As on heating, the internal energy of a substance increases. There are vibrations in molecules and therefore, the molecular distances increase. As the distance between molecules is increasing, the substance occupies more space. Hence, as the temperature is increased so their volume increases.

So, we can generalize that:
With an increase in temperature:
Mass remains unaffected
Volume increases
As, $density = \dfrac{{mass}}{{volume}}$
In the diffraction, the numerator remains constant but the denominator is increasing. As a whole, the diffraction (=density) is decreasing.

So density decreases.
Correct option is (B).

Note: In case of water, the expansion of volume follows a different trend. So the variation in density with temperature will be different.
The variation in density for water can be shown as: