= Solution
Let $p$ be the row player's <probability> of $T$. Its <expected payoffs> against $L$ and $R$ are $6-5p$ and $2+p$. The <security level payoff> is therefore
$$
\max_{0\leq p\leq1}\min\{6-5p,2+p\}.
$$
The first line decreases and the second increases, so their intersection is optimal: $6-5p=2+p$ gives $p=2/3$. Hence
$$
\boxed{\text{Security level }=8/3,\qquad\Pr(T)=2/3,\quad\Pr(B)=1/3.}
$$
The two column actions then give the same <expected payoff>, so an opposing <mixed strategy> cannot reduce it further.
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