For those who still believe it is D, let me ask you another question:
A spaceship travels at a speed of 0.8c relative to the earth.
there are 2 identical clocks, one on the ship and and one on the earth. for the journey, the astronaut observed 6 hours to have elapsed on the clock on the earth. how much time would have elapsed when he looked at his own clock?
the question is clear:
consider the (simplified) lorentz transformation for time:
t' = t/(1-v^2/c^2)^(-1/2)
where t' is the time on the spaceship's frame and t is the time on earth and v is the relative speed between them
we're given that t' = 10, v=0.8C we have to find t.
10 = t/(0.6)
t = 6