Answer
a. The spacecraft.
b. u/c = 0.9968.
Work Step by Step
a. The spacecraft clock measures the proper time $\Delta t_o$. It shows the shorter elapsed time.
You measure a dilated time interval of $\gamma \Delta t_o $.
b. The problem asks for a speed such that the value of $\gamma$ is $\frac{1}{\sqrt{1-u^{2}/c^{2}}}=\frac{0.150s}{12.0\times10^{-3}s}=12.55$.
$u/c=\sqrt{1-(1/12.5)^2}=0.9968$
