The correct option is (d) ends of beam are fixed support
Best explanation: The following assumptions were made while deriving expression for elastic critical moment: (i) beam is initially undisturbed and without imperfections, (ii) behaviour of beam is elastic,(iii) beam is loaded with equal and opposite end moments in plane of web, (iv) load acts in plane of web only, (v) ends of beam are simply supported vertically and laterally, (vi) beam does not have residual stresses.