Answer
$C_1^{'}$
Work Step by Step
$C_1^{'}$ will measure the lowest time out of all three clocks because it is the only one that's in the same reference frame as $S^{'}$. This is important because only frame $S^{'}$ has velocity, Because the other clocks have velocity relative to $C_1^{'}$, time dilation says they each will record a longer time than $C_1^{'}$.
