Question

Which solid have fixed geometric shape. ​

Answer 1

Answer:

(a) Solid O2 has a fixed volume and shape, and the molecules are packed tightly together.

Answer 2

The short answer is “crystallisation”, which ultimately comes down to the direction and force of inter-atomic bonds. There are many very interesting books and academic papers about it. Search “ice rules” for one small corner of the topic.
Another short answer is “because it’s cold enough” for there to be relatively little energy in the elements and compounds that are solid at “room temperature”.
Here’s a thought: Out of the 80-odd reasonably stable elements, we have eleven gases and two liquids. The rest are solids (though francium and cesium will melt on a hot day).
So on a cosmic scale, planet Earth is perishingly cold.
Another thought: many solids don’t hold their shape under even moderate force: they break apart and can be ground into powders - which, though they are solids, behave in many ways like liquids.