Correct option is D
Given:∙Frame S’ moves with respect to frame S at velocity v=0.6c∙In frame S’, a particle has velocity u′=0.4cWe are asked to find the velocity of the particle in frame S, denoted as u the relativistic velocity addition formula:u=1+c2u′vu′+vu=1+c20.4c⋅0.6c0.4c+0.6c=1+0.241.0c=1.241.0c=0.81c