Refer to figure.
Subsidence inversions occur when high surface pressure is present. This brings divergence at the surface, causing the air column above the high pressure to subside. This air warms up at the DALR (Dry Adiabatic Lapse Rate) of 3ºC/1000ft, which means that the air quickly gets quite warm, often quite a bit warmer than the air near the ground. This temperature difference will eventually stop the air descending any further, and instead the air begins to spread outward in divergence, but with a layer of cooler air trapped underneath this inversion that has formed. The inversion usually exists at around 2 - 3 000 ft, and can bring a reduction in visibility due to haze and other particles in the air under the inversion.

Therefore, the location and time of year for this would be anywhere that gets high pressure systems.

• Northern continents in Winter, such as Canada and Siberia tend to get very cold, bringing high pressure, so these are correct. Subtropical ocean areas include the belt of subtropical high pressure, so that is certainly correct. Therefore, this option is correct.
• The Polar regions do have high pressure systems, but less so during the Summer than in the winter.
• The Equatorial regions tend to have low pressure, due to the intense heating, and the location of the ITCZ (which is a product of this low pressure).
• Mountainous regions have very varied weather, but you wouldn't expect high pressure for every mountainous region during every season, so incorrect.

Note that the Encyclopedia Brittanica quotes the following "Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.", so we can see where the question writer got this from.