c
\(\omega=\sqrt{\frac{k_{\text {eq }}}{\mu}}\)

\(\mu=\) reduced mass

springs are in series connection

\(k _{eq}=\frac{ k _{1} k _{2}}{ k _{1}+ k _{2}}\)

\(k _{ eq }=\frac{ k \times 4 k }{5 k }=\frac{4 k }{5}\)

\(k _{ eq }=\frac{4 \times 20}{5} N / m =16 N / m\)

\(\mu=\frac{ m _{1} m _{2}}{ m _{1}+ m _{2}}=\frac{0.2 \times 0.8}{0.2+0.8}=0.16 kg\)

\(\omega=\sqrt{\frac{16}{0.16}}=\sqrt{100}=10\)