Final answer:
The thermosphere expands and atmospheric density decreases when the Sun is hotter, as the atoms in the thermosphere move faster due to increased kinetic energy, aligning with the concept of hydrostatic equilibrium. Option D is correct.
Step-by-step explanation:
The thermosphere expands in size during periods when the Sun is hotter; this corresponds to reducing atmospheric density. As the Sun's activity increases, temperatures in the solar atmosphere rise, particularly in outer layers like the chromosphere and corona. The increase in temperature leads to the individual atoms in the thermosphere gaining kinetic energy and moving faster, which in turn causes the thermosphere to expand.
According to the hydrostatic equilibrium, as the thermosphere warms and expands, the density of the atmospheric gases decreases because the particles are more spread out, so the option (D) Hotter; reducing atmospheric density is correct.