Answer
c
Work Step by Step
All observers measure the speed of a light beam to be c.
Check this. Let the Earth’s reference frame be S and the ship’s reference frame be S’.
The velocity of the ship relative to the Earth is v = +0.70c. The velocity of the light beam relative to the ship is u’=+c.
Use the relativistic velocity addition formula, equation 26–10, to find the velocity of the light as measured by Earth observers.
$$u=\frac{v+u’}{1+\frac{vu’}{c^2}}$$
$$u=\frac{0.70c+c}{1+(0.70)(1)}= c$$