Mountains are considered a constructive force primarily because of the second option you've mentioned: the plates collide and go up causing the build-up of land. This process occurs in areas of tectonic activity, where two tectonic plates collide. The immense pressure and force from the collision can cause the Earth's crust to buckle and fold, leading to the uplift of land and the formation of mountain ranges.
In contrast, while the other processes you mentioned — such as spreading apart and tearing the land or subduction causing one plate to melt — are indeed related to tectonic activity, they don't directly lead to the constructive formation of mountains in the same way that plate collision does.
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