\(T=2 \pi \sqrt{\frac{m}{K}}\)
Time period is maximum when \(K\) is minimum. In \((a), (c)\) and \((d)\) the spring constants are in parallel so the \(K_{ eq }=2 K\). Only in case \((b)\) springs are in series.
So, \(K_{\text {eq }}=\frac{K}{2}\)
Hence time period in this case will be maximum.