1. The problem asks to list the elements of sample space $S$ corresponding to event $E$ where at least two of the three rivers are safe for fishing. 2. Each element in $S$ is a 3-letter string where each letter represents a river's safety: $F$ for not safe (Fail) and $N$ for safe (No fail). 3. "At least two rivers are safe" means the string must have at least two $N$s. 4. Check each option: - a. $\{FFF, FFN, FNF, NNN\}$: $FFF$ has 0 $N$s, $FFN$ has 1 $N$, $FNF$ has 1 $N$, $NNN$ has 3 $N$s. Only $NNN$ qualifies. - b. $\{FFF, FFN, FNF, NFN\}$: $FFF$ 0 $N$s, $FFN$ 1 $N$, $FNF$ 1 $N$, $NFN$ 2 $N$s. Only $NFN$ qualifies. - c. $\{FFF, FFF, FNF, NFF\}$: $FFF$ 0 $N$s, $FNF$ 1 $N$, $NFF$ 1 $N$. None have at least 2 $N$s. - d. $\{FFF, FFN, FNF, NFF\}$: $FFF$ 0 $N$s, $FFN$ 1 $N$, $FNF$ 1 $N$, $NFF$ 1 $N$. None qualify. 5. Therefore, the correct set with elements having at least two $N$s is option b: $\{FFF, FFN, FNF, NFN\}$, but only $NFN$ meets the condition. 6. Since the question asks for the elements corresponding to event $E$, the answer is $\{NFN\}$. Final answer: $\boxed{\{NFN\}}$