Original Pulse is having a frequency of $1MHz$, and $1024$ modulus ripple counter means $10$ JK Flip Flops are connected.
So, $t_{clk} \ge 10*t_{pd}$ ( $t_{pd} \rightarrow$ Propagation delay of $1$ FF)
$\frac{1}{10^6} \ge 10*t_{pd}$
$t_{pd} \le 10^{-7}sec$
$t_{pd} = 100ns$