At
$t=1,\Rightarrow2MSS$
$t=2, \Rightarrow4MSS$
$t=3, \Rightarrow8MSS$
$t=4, \Rightarrow9MSS$ (after threshold additive increase)
$t=5, \Rightarrow10MSS$ (fails)
Threshold will be reduced by $\dfrac{n}{2}$ i.e. $\dfrac{10}{2}=5$.
$t=6, \Rightarrow2MSS$
$t=7 \Rightarrow4MSS$
$t=8, \Rightarrow5MSS$
$t=9, \Rightarrow6MSS$
$t=10, \Rightarrow7MSS$.
So at the end of $10^{th}$ transmission congestion window size will be $8 MSS$.