In zBrush, how do we make a reflective mirror like object on which we can see the reflections of neighbouring objects situated around it ?
Zbrush does not currently have a fully featured ray trace rendering engine capable of calculating in-scene reflections natively. Reflections are usually applied as a material effect to give the illusion of reflection, or composited in.
You will have to export to a third party render engine if you need to render mathematically accurate scene/environmental reflection.