Frame S' moves relative to frame S at 0.60 c in the direction of increasing x. In frame S', a particle is measured to have a velocity of 0.4 c in the direction of increasing x'. The velocity of the particle as measured in frame S is:

Correct option is D

​$$\begin{aligned}&\textbf{Given:} \\&\quad \bullet \text{Frame S' moves with respect to frame S at velocity } v = 0.6c \\&\quad \bullet \text{In frame S', a particle has velocity } u' = 0.4c \\\\&\text{We are asked to find the velocity of the particle in frame S, denoted as } u \\\\&\textbf{ the relativistic velocity addition formula:} \\&\quad u = \frac{u' + v}{1 + \frac{u'v}{c^2}} \\\\&\quad u = \frac{0.4c + 0.6c}{1 + \frac{0.4c \cdot 0.6c}{c^2}} = \frac{1.0c}{1 + 0.24} = \frac{1.0c}{1.24} = 0.81c\end{aligned}$$​​